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Notes Unit 8
Epidemiology
History of Epidemiology
Epidemiology is the study of when and where diseases occur and how they’re transmitted. Public health
programs develop and implement ways to prevent and control disease. In other words, epidemiology generates
the information needed to carry out effective public health programs with the goal of preventing disease. The
distinctive feature of epidemiology is its focus on populations, rather than individuals. Here is a brief timeline
of the field of epidemiology:
1700 – 1713
1847
1854
1900
1926
1955
Bernardino Ramazzini (Italian physician) described the first occupational diseases by
studying the diseases specific to different tradesmen – painters, farriers, etc.
Oliver Wendell Holmes (US physician) and Ignaz Semmelweiss (Austrian physician)
discovered at the same time that puerperal fever occurs when birth attendants do not
wash their hands after examining infected patients or doing autopsies.
John Snow (English physician) showed that cholera epidemics are related to a
contaminated water supply in London
Walter Reed (US surgeon) discovered that yellow fever is transmitted by mosquitoes
Joseph Goldberger (US physician and microbiologist) discovered that pellagra, a
potentially fatal disorder , is caused by a dietary deficiency of niacin.
Evarts Graham (US surgeon) and E. Cuyler Hammon (US statistician) proved that
smoking is a significant risk factor for premature death from lung cancer
Microbes as Agents of Disease
It wasn’t until the 19th century that a link was made between the lack of sanitation and infectious disease. But
microbes had not yet been discovered. It was Robert Koch, a German bacteriologist, who proved that specific
microbes caused specific diseases. It was through these postulates that microbiologists identified many new and
emerging diseases! He developed 4 postulates for proving that a particular organism caused a particular
disease:
1. Association: the causative agent must be present in every case of the specific disease.
2. Isolation: the causative agent must be isolated in every case of the disease and grown in pure culture.
3. Causation: the causative agent in the pure culture must cause the disease when inoculated into a healthy
and susceptible lab animal.
4. Reisolation: the causative agent must be reisolated from the lab animal and be identical to the original
causative agent.
Microbial Mechanisms of Disease
How do microbes cause disease? There are two terms that need to be distinguished from each other: an
infection is the multiplication of microbes on or in the body without producing definitive symptoms; disease is
a possible outcome of infection that results in becoming sick to some degree. Some diseases can be clearly
manifested, while others may be subclinical (meaning that they are asymptomatic). The chance of getting a
particular infection depends on four things:
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1. Dose – the number of microbes to which the host has been exposed. In some instances, it only
takes 10 bacteria to establish infection (tuberculosis); whereas in other cases, it takes 10 million
(Salmonella).
2. Virulence – microbial mechanisms or weapons they use to hurt us. The more virulent (harmful)
the organism is, the less the dose needed to make us sick.
3. Resistance – how good the host immune system is.
4. Portals of Entry and Portals of Exit - these are the sites where the parasite enters and exits the
host. See diagram below for most common ones:
Stages of a Microbial Disease
There are 5 general stages to any disease process:
1. The Incubation Stage – this is the period between the pathogen’s access to the body through a portal of
entry and the display of signs and symptoms. Differs depending on the pathogen. Can be hours to years.
2. The Prodromal Stage – this is the period in which early symptoms appear. Symptoms are usually vague
and are frequently characterized by tiredness, headache, muscle aches, and “feeling lousy”.
3. The Illness Stage (Acme) – this is the period during which the disease is most acute and is accompanied by
specific symptoms.
4. The Stage of Decline – period during which the symptoms gradually subside.
5. The Convalescence Stage – period during which symptoms disappear and recovery is complete.
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Transmission of Disease
Disease can be spread in a variety of ways. Below are some common ways:
i. Direct methods – such as hand-shaking, kissing, sexual intercourse, and contact with feces.
Also can catch by exposure to droplets, such as when sneezing or coughing. Can also have
direct contact with an infected animal.
ii. Indirect methods – consumption of contaminated food or water, through fomites (inanimate
objects that carry disease organisms) such as bed linens, door handles, toilet seats, etc. Can be
transmitted by arthropods (insects). Carriers of disease can have the disease agent, but not be
sick, but can still transmit the organism to another person.
Types of Diseases
Types of Diseases – there are many words that describe the type of disease. A few of them are below:
 Acute – a disease that develops rapidly, is usually accompanied by severe symptoms, comes to a
climax, and then fades rather quickly.
 Chronic – diseases that linger for long periods of time. Symptoms appear slowly, a climax is rarely
reached, and convalescence may continue for months.
 Primary disease – occurs in a healthy body
 Secondary disease – occurs in a weakened individual
 Local disease – restricted to a single area of the body
 Systemic disease – found throughout the body, disseminating to the deeper organs and systems
 Bacteremia – when you have bacteria in the bloodstream
 Septicemia – any kind of infection in the blood
Concepts of Epidemiology
Epidemiology is an investigative methodology designed to determine the source and the cause of diseases and
disorders that produce illness, disability, and death in human populations. Epidemiologists are the “disease
detectives” that figure out how and why outbreaks occur around the world. Factors they consider are age of
population, sex, race, personal habits, geographic location, seasonal changes, modes of transmission, and others.
Epidemiologists focus on the frequency and distribution of diseases in populations and classify diseases as one
of the following:
 Sporadic – those diseases which occur only occasionally and at irregular intervals in a random and
unpredictable fashion. Ex: Tetanus.
 Endemic – diseases which are continually present at a steady level in a population and pose little
threat to the public health. Ex: common cold, chicken pox.
 Epidemic – When there is a sudden increase in the morbidity (illness rate) and mortality (death
rate) above the usual levels and when it causes a potential public health problem. Ex: Bubonic
plague, smallpox.
 Pandemic – diseases which spread across continents and may be worldwide. Ex: AIDS.
With respect to the source and the spread of epidemics, there are two types of spread:
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 Common-source epidemics – those which arise from contact with a single contaminated source and
are usually associated with fecally contaminated foods and water. A large number of people become
ill quite suddenly, and the disease peaks rapidly in the population, such as in food poisoning.
 Propagated epidemics – those which are the result of direct person-to-person transmission; the
microbe is spread from infected individuals to noninfected individuals. The number of infected
people in this type of epidemic rises more slowly and decreases gradually.
Surveillance of disease outbreaks and of factors that could trigger outbreaks is an important aspect of the
mission of public health organizations throughout the world, including the World Health Organization (WHO),
the Centers for Disease Control (CDC) and agencies at the state and local level.
Questions An Epidemiologist Might Ask
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To keep track of these diseases in the United States, physicians are required to report cases of certain diseases,
called “notifiable diseases”, to their local health departments. See list of current notifiable diseases in the table
on the next page:
Nosocomial Infections
A nosocomial infection is an infection that is acquired by a patient during their hospital stay. Estimates are that
2-4 million people each year are infected during a stay in a medical facility. Why does this happen? There are
a number of possible ways:
 Weakened immune systems allow patients to become sick easier
 Invasive procedures, such as surgery, IV’s, etc. allow pathogens to be introduced into patients easier
 Because antibiotics are used often, there may be many strains of antibiotic-resistant bugs that infect
patients
 Hospital staff many transmit bugs from one patient to another unwittingly
 Lack of proper sterilization of surfaces and instruments
What is the breakdown of how nosocomial infections most frequently occur?
 50% are of the urinary tract caused by catheterization
 25% are at the surgical site caused by surgery
 12% are lower respiratory tract diseases caused by respirators and breathing aids
 6% are blood infections, causes unknown
 7% are caused by other types of infections, such as skin infections
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