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Basic Disease Investigation in Colorado
Module 1: Communicable Disease Basics
Introduction
This course is designed for non-epidemiologists who work in public health, environmental health,
and other healthcare fields who wish to expand their knowledge of epidemiology and disease
investigation in order to conduct routine disease control and assist with disease outbreaks within
the community.
This course is the first of six modules that are being developed to enhance disease investigation
and disease reporting in the state of Colorado.
This course will take approximately one hour to complete. When you are finished reviewing the
course information, click on the "Course Exam" link. You MUST complete and pass the course
exam in order to receive credit for taking this course.
This course also contains links to several websites and other useful resources. The "Resources"
link on the home page contains all of the resources referenced throughout this course. The
"Glossary" link provides a list of all defined terms used in the six disease investigation modules.
This course will introduce you to basic communicable disease terminology, measures, and
resources commonly used by practicing epidemiologists in the state of Colorado. This course will
also prepare you to work collaboratively with epidemiologists and public health surveillance
personnel in your region of the state.
After completing this course, you will be able to:
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Identify the three components of the disease triangle of communicable disease theory
Describe at least three routes of transmission and provide one disease example for each
route
Identify two resources for information about communicable diseases
Define basic epidemiology terminology
Communicable Diseases
Communicable, or infectious, diseases are human illnesses caused by viruses, bacteria,
parasites, fungi and other microbes. These diseases may be spread by direct contact with an
infected person or animal, by ingesting contaminated food or water, by insects like mosquitoes or
ticks (vectors), or by contact with contaminated surroundings like animal droppings or even
contaminated air.
With the introduction of antibiotics over 50 years ago, scientists made sweeping predictions
heralding the end of death and suffering from infectious diseases. However, during the past 25
years, microbes have demonstrated their tremendous ability to adapt, survive and challenge us.
Infectious diseases remain the leading cause of death worldwide.
Of the top ten causes of death compiled by the World Health Organization for 2004, five were due
to infectious diseases. In the United States, two of the ten leading causes of death are infectious
diseases (HIV/AIDS and pneumonia/ influenza). The Centers for Disease Control and Prevention
(CDC) reports that 160,000 Americans die each year with an infectious disease as the underlying
cause of death.
This course will introduce you to a variety of communicable diseases, routes of disease
transmission, and disease control terminology.
In order for a disease to be communicable, the disease must be able to be transmitted from an
infected person, animal or an inanimate source to a susceptible host. Transmission can occur
directly or indirectly, through an intermediate animal, plant or object. Three elements must be
present for successful disease transmission:
1.Agent
2.Environment
3.Host
The relationship between the agent, the environment and the host is complex, and is often shown
as a triangle (and referred to as the epidemiologic triangle). If any side of the triangle is
disrupted, disease transmission will likely cease.
Chapter 1: The Agent and Transmission Routes
Have you ever had a stomachache, a bad cough or a sore throat and wondered what was
causing you feel so bad? Well, it's likely that your symptoms were due to one of many different
types of viruses, bacteria, parasites, fungi or other microbes that live in our environment.
In this chapter, you will learn about the different characteristics of disease causing agents.
Another term for a communicable disease-causing agent is pathogen.
Communicable disease agents must be able to do three things:
1. Be present in the environment
2. Find a means of transmission (spread)
3. Survive and multiply
The characteristics of an agent play a large role in determining an agent's odds for survival. In
turn, these agent-specific characteristics determine how the disease will be distributed in the
population and how it will be transmitted.
The following characteristics can aid in improving the agent's survival and ability to infect its host.
Reservoirs
A reservoir is an animal, plant, or substance that harbors a particular organism that can cause
disease. A reservoir may or may not have the disease caused by the organism, but serves as a
place where the organism can live and multiply until such a time that it is transmitted in some way
to the host. An individual who is infected with the organism, and is not showing symptoms, can be
a source of infection to others. This individual is referred to as a carrier. For example, rodents act
as a reservoir for plague, cattle are often reservoirs for E. coli O157, and chickens, reptiles and
amphibians are often reservoirs for Salmonella.
During the World Health Organization (WHO) campaign to eradicate smallpox, ring vaccination
was successful because humans are the only reservoir for infection by the variola virus (the virus
that causes smallpox). When humans around the world were vaccinated, and therefore, not
susceptible to infection, the chain of transmission was ended. Agents with multiple reservoirs,
such as Salmonella, are very unlikely to be eradicated.
Persistence
The persistence of the organism in the environment determines where and for how long we can
expect to find the agent. For example, anthrax (Bacillus anthracis) produces spores that can live
in the environment for many years. In contrast, Campylobacter (one of the most common
bacterial causes of diarrheal illness in the United States) prefers a low-oxygen environment, so
Campylobacter doesn’t survive for very long on surfaces exposed to the air.
Latency and Incubation Period
Most organisms need some time from the moment they infect a new host until they can become
infectious and be spread further. The latent period is the period of time between when the host is
infected to when he or she becomes infectious. This period can vary from a few days in the case
of influenza, to several weeks for hepatitis A.
A closely related concept is the incubation period. The incubation period is the time interval
from the time an agent enters the host to the time the agent starts to cause disease.
The interplay between these 2 periods is crucial for disease control efforts. For many diseases,
the latent period is slightly shorter than the incubation period, meaning that people can be
contagious before showing any symptoms of illness and isolating people once they become ill will
not necessarily prevent transmission. This is true with many respiratory viruses and with hepatitis
A. These characteristics help the agent to continue to survive and move to new hosts.
Vector
A vector is an organism, such as a mosquito or tick that carries disease-causing microorganisms
from one host to another. This can occur by direct inoculation through skin penetration, as with
biting insects, or through mechanical transmission where the vector moves the organism from
one place to another, as when Shigella is passed from feces to food on flies' feet.
Vehicle
A vehicle is a contaminated inanimate object that can be an intermediary between a reservoir and
a susceptible host. Vehicles can include food, water, objects such as toys or bedding, medical
equipment, and the hands of health care workers. An agent’s ability to persist in the environment
will determine whether the agent can survive on a vehicle(s), and if so, which objects are suitable
for transmission.
Intermediate Host
Some organisms survive in their early developmental stage in another host before infecting the
final host. For example, most parasite trematodes (flukes) and cestodes (tapeworms) have
indirect lifecycles requiring intermediate hosts.
One such example is the pork tapeworm. Pork tapeworm eggs are ingested by a pig, grow into
encysted larvae, are consumed by a human, and then the cyst wall is dissolved in the human's
intestine, freeing the larvae to attach to the intestinal wall and grow into an adult worm.
Factors That Affect the Agent's Ability to Cause Disease
1. Pathogenicity: the ability of an organism to produce disease, that is, signs and symptoms of
illness. One way to measure the pathogenicity of an organism is:
The total number of people who become ill
The number of people who were infected with the organism
2. Virulence: the degree to which the organism causes severe illness. Virulence for a particular
agent can vary depending on the organism’s subtype or serotype. For example, Salmonella
serotype Newport usually causes a diarrheal illness, whereas Salmonella serotype Typhi
causes a more severe, systemic illness called typhoid fever. One measure of virulence is the
case fatality ratio, which is the number of deaths divided by the total number of people ill.
3. Toxicity: some agents produce toxic proteins that elicit an extreme response in the host. An
example of this is botulism, which is caused by ingestion of botulinum toxin. The toxin can be
produced when food containing the C. botulinum bacteria is mishandled. Botulism is not
actually an infection, but rather an intoxication.
4. Infective Dose: the number of organisms necessary to overwhelm the immune system and
produce disease. Infective dose varies according to species and strain - some agents require
a very high number of organisms to be present in order to overwhelm the host's immune
system, other agents only require a few. An organism with a high infective dose, as in the
case of Vibrio cholerae, responds well to improved sanitation, because in order for a person
to become ill, the person must come in contact with many organisms before they are infected.
An organism with a low infective dose, such as Coxiella burnetii, the agent that causes Q
fever, can be passed by a few organisms. A person can become infected with Q fever by
simply being present at the birth of an animal infected with the organism.
Types of Transmission
Transmission of an agent to a host can occur by direct or indirect transmission.
Direct Transmission
Direct transmission is the direct movement of an agent from a reservoir or infected host to a
portal of entry in the new host. Examples of direct transmission include:
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Person-to-Person Transmission: touching, kissing, or sexual activity
Droplet Transmission: droplets from a cough or sneeze are projected directly into the air and
onto the mucous membranes of a host.
Biting (human or animal bites)
Note: People can be "carriers" of certain diseases, which means that they do not show symptoms
of illness but they can effectively transmit the disease to others. Review the text below to learn
more.
Carrier or Subclinical State
People can be "carriers" of certain diseases, which means that they do not show symptoms of illness but they
can effectively transmit the disease to others. Since only a portion of the ill population exhibits symptoms with
these types of illnesses, the number of clinically ill is not the same as the total number of people who are
actually infected. The proportion of infected people who have symptoms, varies greatly be disease. The
carrier state can be either transient, or persistent and chronic.
The carrier state can manifest in the following ways:
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The person is asymptomatic (showing no symptoms) while infected, but is infectious.
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The person is still in the incubation period for illness, and will develop symptoms at a later date, but is
already infectious.
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The person is infectious after symptoms have resolved.
For example, people infected with hepatitis A are infectious for approximately 2 weeks before they develop
symptoms (during the incubation period) and continue to be infectious for approximately one week after they
develop jaundice (a symptom of hepatitis A). The period of greatest communicability is during the incubation
period before symptoms develop. In children, HAV infections often may not result in clinical disease, but they
can still transmit infection, especially during diaper changes, as hepatitis A is spread through fecal-oral
transmission.
Indirect Transmission
Indirect transmission occurs when an agent moves from a reservoir or host to another host by
means of a living organism or an inanimate object. Examples of indirect transmission include:
Vehicle borne Transmission
Transmission via a contaminated inanimate object, such as food, water, objects such as toys or
bedding, medical devices, etc. The organism may or may not multiply in or on the vehicle. Some
common vehicle borne infections are foodborne illness resulting from food prepared by an ill food
handler or nosocomial (or hospital acquired) infections that are a result of poor hand-washing
among hospital staff.
Vector borne Transmission
Transmission via a living object (vector). Examples of vector bone transmission include infection
with West Nile virus and plague. Indirect transmission through the bite of a flea is the most
common route of transmission between plague-infected rodents and humans.
Airborne Transmission
Transmission via droplet nuclei, or aerosols, that remain airborne for long periods of time and are
inhaled. Examples of diseases spread by airborne transmission include measles and
tuberculosis. In contrast, droplets from coughs and sneezes are heavy and settle quickly, so
transmission of agents contained in droplets is considered “direct.”
Types of Diseases
Diseases can be categorized based on the way that they are transmitted. Review the following
table to learn about the different ways that communicable diseases can be transmitted (as well as
how they can be prevented or controlled). Note: Some diseases fall into more than one of these
categories.
Transmission Route
Control Methods
Fecal-Oral
(Yes, that means precisely what you think it means)
Fecal-Oral: Control Methods
The most important control methods for fecal-oral
diseases are adequate hand-washing, and excluding
ill people from preparing food until they are no
longer contagious. It is also important to ensure that
water is clean and comes from a safe source. Some
of these diseases, such as hepatitis A and polio are
vaccine-preventable.
Hosts (human or animal) infected with these agents
shed the agent in their feces. Infection can occur
through direct person to person contact and
ingestion of contaminated food and water. Such
diseases often cause diarrhea and can include
giardiasis, shigellosis, E. coli 0157 infection and
cholera.
Excreted load, also referred to as shedding, refers to
the number of organisms shed in urine or feces, and
is an important factor in transmission. It is unrelated
to the severity of symptoms.
Airborne and Droplet-borne Diseases
Many of these diseases produce either respiratory or
skin rash symptoms. The difference between
airborne and droplet transmission is in the size of
the particles expelled. Droplet transmission occurs
by droplets greater than five microns. The droplets
are sprayed into the air, but usually fall short of three
feet. Exposure does not generally occur beyond this
range. Diseases spread by droplet transmission
include influenza, pertussis, and mumps.
Aerosolized droplet nuclei smaller than five microns
may remain in the air longer than larger droplets,
increasing the range at and length of time during
which people can be exposed. Diseases spread by
airborne transmission include measles and
tuberculosis.
Airborne and Droplet-borne Diseases: Control
Methods
Control of droplet and airborne infections is difficult,
but covering the mouth in the crook of the arm when
coughing, and washing hands after coughing,
sneezing, or after contact with mucus membranes,
and avoiding people who are sick, a practice known
as ‘social distancing’, can prevent or reduce
transmission. Some of these diseases can be
prevented with vaccination, such as measles,
smallpox, chickenpox, pertussis, diphtheria,
influenza, etc.
Review the CDC guidelines for respiratory
hygiene in healthcare settings.
Infections like influenza or the common cold can be
passed either through respiratory secretions, or by
touch contact from mucus membranes. The infective
dose and the degree of exposure play a large role in
the transmission of these diseases. The force with
which particles are expelled also influences their
communicability, such as with pertussis. Close
contacts are more likely than others to become
infected. In addition, host factors like nutrition status
also play a large role in determining whether a
contact can resist infection.
Blood-borne or Bodily Fluid Transmission
Diseases in this category can be transmitted through
blood, serum, saliva, seminal or vaginal fluid. In
addition to direct transmission, however, indirect
transmission can occur through contact with fomites
exposed to bodily fluid, depending on the hardiness
of the organism. Infective dose also plays some role
in transmission.
For every 100 persons exposed to HIV, one person
will be infected (1:100), for every 10 persons
exposed to hepatitis C, one persons will be infected
(1:10). This is because Hep C is more pathogenic
(disease causing) is shed in larger quantities.
Blood-borne or Bodily Fluid Transmission:
Control Methods
Eliminating or reducing exchange of blood and
bodily fluids eliminates or reduces the risk of
transmission of blood and fluid-borne diseases. This
can include practices such as not sharing
toothbrushes, razors, needles or injection
equipment. Sexual transmission is trickier to control.
Sexual transmission can be controlled by either
abstinence, or sex with a barrier, such as a latex
condom.
Some diseases can be spread by all bodily fluids,
while some can only be spread by blood.
Vector-borne Diseases
This set of diseases is transmitted through another
organism called a vector, since it uses a transporter
to get from one place to another, (a "Light Rail" for
microbes). Vectors are often insects.
This method of transmission eliminates the need for
multiple organisms to scatter throughout the
environment, and is carried straight to the host.
Some organisms go through a developmental stage
in the vector, and others do not. In the infective
stage, often the organism can continue multiplying
and being transmitted for the remaining life of the
vector. Often the organisms are transmitted by bite.
Vector-borne Diseases: Control Methods
The best method of prevention and control is often
to eliminate or reduce the numbers of the vector,
such as draining or eliminating standing water, or
encouraging citizens to reduce potential breeding
sites (as is recommended for mosquito-borne
illness). In addition, it is sometimes possible to limit
the exposure of people to the vector, such as by
covering exposed skin at dusk and dawn, the
feeding times of Culex mosquitos, or wearing bug
repellant.
Diseases are also categorized in other ways, such as by the types of reservoirs where the
causative agents are found, by the strategies used to prevent them, or by the types of illnesses
that they can cause. Some examples of these include:
Zoonotic Diseases
Zoonotic diseases are diseases caused by agents with animal reservoirs. Some of these diseases are
transmitted through a vector, such as plague, and others are not, such as anthrax.
Humans are "accidental hosts" in this cycle, meaning they are not natural reservoirs for the agents in
question but are incidentally infected when they have contact with those reservoirs. Other zoonotic
diseases can be transmitted through contamination of food or water by animal feces (e.g. E. coli O157),
contact with urine or animal hides (e.g. hantavirus or anthrax), or animal bites (e.g. rabies).
Food-borne Diseases
Foodborne illness occurs through consumption of contaminated food. Food contamination can be the
result of many factors including: food that was prepared by an ill food handler, food that was not cooked
thoroughly before consumption (such as raw oysters or undercooked chicken), food that was held at
improper temperatures allowing toxins to be produced, or preparing food on contaminated surfaces.
Foodborne illness can be an intoxication, poisoning by toxins (e.g. Staphyloccus aureus toxin or
Clostridium botulinum toxin), or infection with bacteria, viruses, or parasites (e.g. Campylobacter,
norovirus, or Cyclospora). Food-borne diseases can be intertwined with water-related diseases, since food
preparation often involves water.
Water-borne Diseases
These diseases are passed through contact with, or ingestion of, contaminated water. Cryptosporidiosis
and giardiasis can occur naturally in Colorado through the ingestion of contaminated water. Wound
infections with Vibrio species can occur among people with wounds expsosed to seawater. In addition,
water contaminated with feces can harbor organisms that contaminate food sources, such as raw shellfish,
which can pass along the hepatitis A virus. Disasters, such as Hurricane Katrina, can cause widespread
contamination of water by sewage, and a proliferation of water-related diseases not typically found under
other circumstances.
Vaccine-Preventable Diseases
This refers to the group of diseases for which a key prevention strategy is vaccination. This group includes
diseases that are spread through many different routes of transmission, including: airborne transmission
(e.g. measles), droplet transmission (e.g. pertussis, influenza), fecal-oral transmission (e.g. hepatitis A,
polio), and blood-borne/body fluid transmission (e.g. hepatitis B).
Chapter 2: The Host and the Environment
The Host
The host is an organism that is capable of being infected or affected by a disease agent. In some
circumstances, an agent and its host can live together in reasonable harmony, for example
humans and herpes simplex virus. In others, the agent causes the host to become very ill or
possibly die, such as with plague.
In the human host, the agent, or pathogen, causes disease by either disrupting a vital body
process or stimulating the immune system to mount a defensive reaction. An immune response
against a pathogen can lead to a variety of symptoms.
The likelihood that an individual will become infected and develop illness depends on a variety of
“host factors,” or factors that are specific to that individual including the host’s susceptibility to the
disease in question and the host’s overall resistance.
Host Factors Affecting Exposure
There are many behavioral and sociodemographic factors that may affect whether a host is more
or less likely to be exposed to an agent in the first place. Some of these include things such as:
• Occupation
• Travel
• Eating habits
• Socio-economic status
When performing epidemiologic investigations, disease investigators sometimes find a correlation
between the amount of agent to which the host is exposed and the strength of the illness, or the
likelihood of becoming ill. This relationship is known as dose-response. Dose-response can be
very apparent in certain kinds of foodborne illness (the more you eat, the more likely you are to
become ill).
Susceptibility
Many factors such as age, gender, and conditions such as pregnancy can affect a person’s ability
to fight off, or their resistance to, infection. Before we get to all those factors, one key host factor
is the concept of susceptibility. A person is susceptible to a disease if he or she is not immune to
that disease. Agents can only infect people who are susceptible. For example, persons who have
been vaccinated against pertussis or have already had pertussis, cannot catch pertussis or pass
it to others. Individuals who have not previously had pertussis, or who have not been vaccinated
recently, are susceptible to infection.
Immunity
Immunity to infection can occur in a number of ways:
Passive Immunity
The host acquires antibodies to the agent from another source. Babies acquire maternal
antibodies in utero and through breast milk. Passive immunity is also conferred by inoculation
with immune globulin. Protection from disease is generally temporary.
Active Immunity
Active immunity entails either the elicitation of an immune response through intentional
inoculation with the agent by vaccination, or by contracting the infection. Someone with active
immunity will be able to mount an immune response when exposed to an agent.
Herd Immunity
Both passive and active immunity contribute to the overall ‘herd’ immunity of a population. When
many persons in a population are immune to an infection, they break the chain of infection by not
transmitting it to persons with no immunity.
Another important concept is that for some diseases, immunity may wane over time,
necessitating “booster” doses of vaccine in order to remain immune. Some examples of this are
tetanus, where adults are advised to receive booster shots every 10 years, and pertussis, where
immunity from childhood vaccinations wanes in the adolescent years.
Resistance
In a population of susceptible people who are exposed to a disease, certain people are more
likely to become ill than others. This is due to each individual's resistance. Factors that affect a
person's resistance to disease include:
Physical Defense Mechanisms
The human body has developed many physical defense mechanisms to protect against disease.
Our skin is an effective barrier against infection, as are mucus-secreting membranes and the
inflammatory response that sends blood and phagocytes to a cut, blister or other area of
inoculation. If these mechanisms are damaged, a person increases his/her risk for infection.
Factors Affecting Overall Immune System Functioning:
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Nutritional status
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Chronic medical conditions/immunosuppressive conditions
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Immunosuppressive drugs
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Age
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Gender
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Pregnancy
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Genetics
The Environment
Environmental factors have a large impact on the disease transmission relationship between the
agent and the host. Environmental factors include living conditions, climate, societal norms,
cultural practices, personal resources, education, biological organisms present, and more.
For example, factors such as temperature, humidity, and wind can affect the transmission
potential and the survival of pathogenic organisms. These factors can also affect the stress level
placed on people, thus increasing a person's susceptibility to disease.
Other examples of how the environment can impact the transmission of disease include:
Methicillin resistant Staphylococcus aureus (MRSA)
MRSA is transmitted from person to person through direct skin contact and contact with
contaminated objects. During the past few years, correctional facilities have seen an increase in
MRSA infections, and even some outbreaks among inmates. Several reasons for the higher
rates of MRSA infections in correctional facilities include crowding, decreased access to soap or
towels, and relatively high rates of pre-existing medical conditions among inmates. This is an
example of how environmental factors particular to a correctional setting affect disease
transmission.
E. coli 0157
E. coli 0157 has a clear seasonal distribution, with cases peaking each year during the summer
months. Several potential reasons for this include increased prevalence of E. coli O157 in
animals (the reservoir) during warmer months, more people coming together for picnics and other
gatherings where meat may not be thoroughly cooked, and warmer temperatures allowing for
increased bacterial multiplication in contaminated foods that are not promptly refrigerated.
Environmental factors are taken into consideration by epidemiologists when they collect
information based on person, place, and time. These factors help to answer questions such as:
• Who is getting sick?
• Where are they getting sick?
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When?
Epidemiologists collect person, place and time information in order to gather clues about the
agent (incubation period), the host (age or sex distribution) and the environmental factors at play
(seasonality, location)
You will learn more about disease investigation and the study of epidemiology in the second
online course, The Basics of Public Health Surveillance.
Chapter 3: General Communicable Disease Concepts
Infection Timeline
Communicable diseases often follow along an epidemic timeline involving the incubation period
and the infectious period. These two terms are defined below using influenza and hepatitis A as
examples:
Incubation Period: Time of infection to clinical disease
Infectious Period: Period during which agent can be transmitted. This can vary depending on the agent.
As depicted in the graph above, the infectious period and the incubation period vary to a large
degree from agent to agent and need to be considered when planning control measures for each
agent.
Scope of Disease
The degree to which a disease exists in a community or area defines whether it is a(n):
Endemic
Endemic refers to the usual level of disease in a population, which can be either a high or low
rate. Chlamydia, which causes pelvic inflammatory disease, while sometimes surfacing as an
epidemic, is often present in the population, particularly among sex workers.
Epidemic
An epidemic is occurring when the amount of disease in a particular population in a particular
time frame is greater than expected. The terms epidemic and outbreak can be used
interchangeably, although outbreak is sometimes preferred in working with the public because the
connotation does not seem to be as frightening.
This graph shows the number
of cases of Salmonella
Newport reported in Colorado
from 1998-2003. In July 2002
and January 2003, the
number of reported cases
exceeded historical averages
and outbreak investigations
were conducted.
Pandemic
A pandemic is an epidemic that crosses the borders of multiple countries at the same time. Three
influenza pandemics occured in the 20th century - 1918, 1957-58, and 1968-69. Currently, the
H5N1 avian influenza virus is being monitored to determine if this virus is going to become
transmitted easily from person-to-person. As an avian disease, the H5N1 virus has already
spread to multiple countries. This disease will not be defined as a pandemic until it becomes
easily transmitted from person-to-person and spreads to multiple countries.
Types of Epidemics (Outbreaks)
Different agents and different exposures result in different types of outbreaks. An epidemic curve,
which shows number of cases over time, looks different for each type.
Point Source Outbreak
A point source outbreak results from a single exposure, such as a contaminated batch of meat. It
appears as a tight clustering over time. The upslope is sudden, and the downslope tapers.
Onsets times/dates for most cases will be within the span of one incubation period for the
disease.
In point source outbreaks as in other types, there
can be secondary waves of infection, as the original
infected persons spread it to others, such as in a
hepatitis A outbreak.
Continuing Common Source
There can be a continuing common source, as with a contaminated well. If the source is
removed, the down-slope will be sharp, if not it will taper as the number of susceptible individuals
declines.. The rise in the curve will plateau rather than demonstrate a peak.
*The cholera outbreak that made John Snow famous would have looked something like this,
except with a sharp downslope when he removed the pump handles.
Propagated Outbreak
This type of outbreak demonstrates transmission from host to host, such as in chickenpox. There
may be several generation periods for the agent.
It demonstrates a gradual upslope as the disease is transmitted person-to-person, and then a
sharp down-slope as the agent runs out of susceptible hosts.
Diagnosis
To diagnose a disease, various tests can be performed depending on the type of agent. Many
organisms can be diagnosed using several different tests. Some of the more common tests are
listed below:
Cultures
Microbiological cultures can identify types of bacteria, such as Staphylococcus or Streptococcus.
In a culture, a growth medium is provided for a particular agent so that the agent can reproduce
and be identified in a laboratory setting. Conditions of public health concern that are frequently
diagnosed by culture include Salmonella, Streptococcus pneumoniae, and Neisseria meningitidis
(the organism that causes meningococcal disease).
Serology
Certain agents cannot be cultured, including most viruses. Serology involves detecting the
antibodies against an infectious agent in the patient's blood. If a person has the antibodies to a
specific virus in their blood, they have been exposed to, and are potentially infected with the virus
in question. Conditions of public health concern that are frequently diagnosed by serology
include: hepatitis A, B and C, rubella and measles.
Polymerase Chain Reaction (PCR)
A more recent development is direct detection of viral proteins and/or bacterial DNA in human
blood or secretions. This can be done by PCR (polymerase chain reaction), involving the
amplification of an agent's genetic material and its subsequent detection with anti-DNA probes. In
the current political climate, with emphasis placed on the possibility of biological warfare,
understanding differences between the presentation of two similar diseases has become more
critical (such as between smallpox and chickenpox, or tularemia and plague). Recognizing the
symptoms of Category A or B agents, which may not be commonly seen, depending on the
setting, is of prime importance in stopping transmission early. Since control measures are highly
specific and must be tailored to the agent, it is very important to identify the agent causing
disease so that correct and specific control measures can be implemented.
Chapter 4: Prevention and Control
The goal of communicable disease epidemiology is to enhance disease prevention and control to
decrease the burden of illness. The most important feature of both prevention and control is to
interrupt the chain of transmission.
For different diseases, this process requires different measures which can range from washing
hands, to turning over trash cans to prevent water from collecting (which destroys vector breeding
sites), to vaccinating humans and/or animal reservoirs, to changing the pattern of intimate human
behavior.
Control measures are highly dependent on the agent in question. Public health professionals
must take many factors in to consideration, including the organism’s reservoir, routes of
transmission, the prevalence of the disease, the presence of a carrier state, and the incubation
period.
Using our disease "bugs" as an example, specific disease control measures include:
Pertussis
Since pertussis is spread via direct person-to-person transmission, disease control should focus
on cough etiquette, exclusion from work or school, and antibiotic therapy. The primary disease
control strategy is vaccination. Pertussis does not have an animal reservoir, that is, it lives only
in humans, so all control strategies focus on people.
Salmonella
To control the spread of Salmonella, epidemiologists recommend several options. These include
hand-washing (after using the bathroom, after animal contact, and before handling food),
excluding infected food handlers from work, encouraging children and the elderly to avoid
animals that are reservoirs (animals in petting zoos, chicks and reptiles, in particular), proper
food handling and cooking food to a proper temperature. These recommendations take into
account that people of different age groups are differently affected by Salmonella infections.
West Nile Virus
To control the spread of West Nile Virus, people should protect themselves by covering exposed
skin with clothing and wearing mosquito repellent that contains DEET. It is also important to
eliminate mosquito breeding sites by removing stagnant, standing water by draining rain barrels,
bird-baths, swimming-pool covers, eavestroughs, flowerpots, and planters. These strategies
take into account control of the vector (the mosquito), but not control of the reservoir (birds),
which would be impractical. They also target environmental factors such as host behaviors that
decrease opportunities for transmission.
Prevention Measures
Many infectious diseases can be prevented through simple and inexpensive methods:
Wash Your Hands
Wash hands before, during and after preparing food, before eating, after using the bathroom or
changing diapers, and after handling animals or animal waste.
Routinely Clean and Disinfect Surfaces
Cleaning with soap and water removes dirt and most germs. Using a disinfectant kills additional
germs. It is important to thoroughly clean areas where germs are likely to be transmitted, such as
the kitchen and bathroom.
Handle and Prepare Food Safely
Buy and refrigerate perishable foods quickly. Store food properly. Don’t allow juices from meat,
seafood, and poultry or eggs to drip on other foods. Wash hands and kitchen surfaces and
utensils while preparing food. Wash raw fruits and vegetables. Don’t eat raw eggs. use a meat
thermometer when cooking meat and poultry.Use different dishes for raw foods and cooked
foods. Keep cold foods cold and hot foods hot. Don’t leave leftovers out longer than two hours.
Get Immunized
Children, adolescents and adults need immunizations. Make sure the members of your family get
the right vaccines at the right time. Keep immunization records for the whole family. Visit
Colorado's Immunization web site to learn more.
Use Antibiotics Properly
Unnecessary antibiotics can be harmful and, if misused, can cause bacteria to become resistant
to treatment. Antibiotics don’t work against viruses like colds and flu. Use antibiotics exactly as
prescribed by your provider.
Summary
In summary, there are many types of communicable diseases that threaten the health of
Colorado communities. In order to be communicable, a disease must be present in the
environment, find a means of transmission and survive and multiply in a host. Simple prevention
measures such as vaccination and hand washing can interrupt disease transmission in order to
keep members of our communities healthy.
For additional information, review the course "Resources" page. This document provides a list of
websites and other useful information related to disease surveillance in the state of Colorado.
To complete this course, you must take the final exam online.