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Chapter 14
Principles of Disease and Epidemiology
Pathology, Infection, and Disease
• Disease is an abnormal state in which part or all of the body is not
properly adjusted or is incapable of performing normal functions.
• Pathology is the scientific study of disease.
• Etiology (cause)
• Pathogenesis (development)
• Structural and functional changes brought about by disease with their final
effects on the body
• Infection is the invasion and growth of pathogenic microorganisms in
the body.
Pathology, Infection, and Disease
• Infection disease
• Presence of particular microorganism in part of the body where is not usually
found.
• When a microorganism overcomes the body’s defenses, a state of disease
results.
• Pathogens - disease-causing microorganisms.
• Pathogenic microorganisms have special properties that allow them to invade
the human body or produce toxins.
• A host is an organism that shelters and supports the growth of
pathogens.
Normal Microbiota
• Animals, including humans, are usually germfree in utero (used in
biology to describe the state of an embryo or fetus )
• Microorganisms begin colonization in and on the surface of the
body soon after birth
• Normal microbiota - microorganisms that establish permanent
colonies inside or on the body without producing disease.
• Staphylococcus on epidermis and mucus membranes;
• Escherichia coli in colon
• Transient microbiota are microbes that are present for various
periods and then disappear.
• Although these types of organisms may colonize a host and reproduce briefly,
the defenses of the human body do not allow them to become permanent.
Relationships Between the Normal microbiota and the Host
• Symbiosis - living together - long-term interaction between two or more
different biological species
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The three types of symbiosis
E.coli (in colon)
Synthesize vitamin K
Mycobacteria
(in the ear)
Disease-causing
bacteria
Figure 14.2
• Normal microbiota and the host exist in symbiosis
• Opportunistic pathogens
• Opportunistic pathogens do not cause disease under normal conditions (live in their
normal habitat) but cause disease under special conditions.
• Streptococcus pneumoniae
• E.coli
Normal Microbiota and the Host
• Locations of normal microbiota on and in the human body
• Human body
is built by 10
trillion cells
• 100 trillion nonpathogenic
bacteria/microorga
nisms that reside
in or on a human
body
• Human body/Commensal microbiota exist as a community
Figure 14.2
Normal Microbiota or Microbiome
• Microbiome - collective genomes of all the microorganisms
(bacteria, fungi and viruses) that reside in a person, literally share our
body space
Microbiome and human body environment
• Microbial community vary at different body sites on the same person (different
ecosystem)
• Bacteria of a specific body site from different persons have more common
characteristic (common ecosystem).
• The microbiome of a healthy and diseased person looks very different (rearranged
ecosystem)
Microbiome and metabolic activity
• Gut microbiome – “forgotten organ”
• Produce hydrolytic enzymes - help our digestive system – they can vary
• Synthesize vitamins - B2, B12 and folic acid – slight variation
• Skin and gut microbiome
• Stabilize epithelial homeostasis and barrier function
• Modulate our innate and adaptive immunity
• Prevent growth of pathogens
• Production of antimicrobial substances
• Compete for the nutrients
Who we are ?
Individual
genetic
material
Microbiome
(bacterial
Phenotype)
Homeostasis
Health
Low risk of allergies
Infection resistance
-------------------------------------------What we eat
(diet)
Perturbation
Diseases
Allergies
Metabolic syndrome
Obesity
Infections
What is a healthy Microbiome?
One size does NOT fit all - Each person’s microbiome is unique
Relationships Between the Normal Microbiota and other
microorganisms
• Microbial antagonism.
• Normal microbiota can prevent pathogens from causing an infection by
competing for nutrition.
• Produce substances harmful for invading organisms
• E. coli produce bacteriocines
• Affecting condition as pH or available oxygen
• Cooperation Among Microorganisms
• In some situations, one microorganism makes it possible for another to cause a
disease or produce more severe symptoms
• Streptococci that colonize the teeth
How to take care of our “second genome”
• We are what we eat
• Probiotics – application of life microbial culture
• Typically lactic acid producing bacteria ingested to aid
digestion and protect intestine from pathogens
• Prebiotics - refer to chemicals that induce the growth
and/or activity of commensal microorganisms
• Mix of bacteria instead of antibiotics
• Fecal transplantation
• Restoration of normal microbiota
The Etiology of Infectious Diseases
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Koch’s postulates are criteria for establishing that specific
microbes cause specific diseases.
Koch’s postulates have the following requirements:
1. The same pathogen must be present in every
case of the disease;
2. The pathogen must be isolated in pure culture;
3. The pathogen isolated from pure culture must
cause the same disease in a healthy, susceptible
laboratory animal;
4. The pathogen must be isolated from the
inoculated laboratory animal and must be shown
to be the original organism
Exceptions to Koch’s Postulates
• Koch’s postulates are modified to establish etiologies of diseases
caused by viruses and some bacteria, which cannot be grown on
artificial media.
• Some diseases have unequivocal signs and symptoms.
• Tetanus - affects the nervous system and causes painful, uncontrolled muscle
spasms
• Some diseases may be caused by a variety of microbes.
• Pneumonia and nephritis
• Some pathogens, cause several different diseases.
• Streptococcus pyogenes – sore throat, scarlet fiver, skin infections,
• Certain pathogens, such as HIV, cause disease in humans only.
Classifying Infectious Diseases
1. Patient Appearance
• Every disease alters body structures and functions
• A patient may exhibit:
• Symptoms (subjective changes in body functions)
• Pain or body discomfort
• Signs (measurable changes)
• Fever, swelling, paralysis
• A physician uses signs to make a diagnosis (identification of the
disease)
• Syndrome - a specific group of symptoms or signs that always
accompanies a specific disease
Classifying Infectious Diseases
2. Spreadability
• Communicable diseases are transmitted from one host to another.
• Chicken pox, genital herpes, tuberculosis
• A contagious disease is one that is easily spread from one person
to another.
• Chicken pox
• Noncommunicable diseases are caused by microorganisms that
normally grow outside the human body and are not transmitted
from one host to another
• Tetanus - Clostridium tetani
Classifying Infectious Diseases
3. Occurrence of a Disease (frequency of occurrence)
• Sporadic – occur only occasionally in population
• Typhoid fever
• Endemic – constantly present in a population
• Common cold
• Epidemic – if many people in a given area acquire a certain disease in a relatively
short period
• Influenza
• Pandemic – An epidemic disease that occurs worldwide
• Some influenza, AIDS
• Disease occurrence is reported by:
• Incidence - Number of new disease cases per population at risk ( indicator of the
spread of the disease)
• Measured over a given time interval
• Prevalence - Number of current ( old and new) cases per population at risk
(
how seriously and how long the disease affects a population)
• Fraction of a population having a specific disease at a given time
• Herd immunity is the presence of immunity to a disease in most of the
population.
• Vaccination (immunization) prevent spread of the disease
Classifying Infectious Disease
4. Severity and Duration of a Disease
• The scope of a disease can be defined as:
• Acute – develops rapidly but last for short time
• influenza, common cold
• Chronic - develop slowly, continual or recurrent for long time
• tuberculosis
• Subacute – intermediate between acute and chronic
• some brain diseases, endocarditis
• Latent – the causative agent remains inactive for a time but than
becomes active to produce symptoms of the disease .
• shingles
Classifying Infectious Disease
5. Extent of Host Involvement
• Local infection: Pathogens are limited to a small area of the body
• abscesses
• Systemic infection: An infection throughout the body by blood and/or lymph
• Bacteremia: Bacteria in the blood
• Sepsis: Toxic inflammatory condition arising from the spread of microbes,
especially bacteria or their toxins, from a focus of infection
• Septicemia: Growth of bacteria in the blood
• Toxemia: Toxins in the blood
• Viremia: Viruses in the blood
• Focal infection: Systemic infection that began as a local infection
• Local tooth infection can moves via blood or lymph to set up a new infection at
another site - rheumatoid arthritis
• Primary infection: Acute infection that causes the initial illness
• Secondary infection: Opportunistic infection after the primary infection
(predisposing) has weakened the body defense
• Subclinical infection: No noticeable signs or symptoms (inapparent infection)
Predisposing Factors
• A predisposing factor is one that makes the body more susceptible
to disease or alters the course of a disease.
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Gender (short urethra in females)
Climate and weather
Age
Fatigue
Inadequate nutrition.
Inherited traits, such as the sickle-cell anemia (SCA)
Chemotherapy
Patterns of Disease
• Development of Disease
Figure 14.5
1. The incubation period is the time interval between the initial infection and
the first appearance of signs and symptoms.
2. The prodromal period is characterized by the appearance of the first mild
signs and symptoms.
3. During the period of illness, the disease is at its height, and all disease
signs and symptoms are apparent.
4. During the period of decline, the signs and symptoms subside.
5. During the period of convalescence, the body returns to its predeceased
state, and health is restored.
The Spread of Infection
1. Reservoir of Infection - A continual source of infection
• Human reservoirs of infection - People who have a disease or are carriers of
pathogenic microorganisms.
• Sick people = actively ill
• Carriers = one who harbors disease organisms in their body without
manifest symptoms (healthy)
• Latent infection carriers = contagious during incubation period or
convalescent period
• Zoonoses - are diseases that affect wild and domestic animals and can be
transmitted to humans.
• Direct contact with animal or its waste
• Eating animals
• Bloodsucking arthropods
• Nonliving reservoirs - Soil, water, and food can be reservoirs of infection
• Presence of microorganisms is often due to contamination by feces or
urine.
The Spread of Infection
2. Transmission of Disease
• Direct contact - involves close physical contact between the source of
the disease and a susceptible host.
• Indirect contact - transmission by fomites (inanimate
objects)
• Vehicle transmission -transmission by a medium such as
water, food, or air.
• Droplet transmission - transmission via saliva or mucus in
coughing or sneezing
• Airborne transmission refers to pathogens carried on water
droplets or dust for a distance greater than 1 meter.
• Arthropod vectors carry pathogens from one host to another
• mechanical transmission passively carry pathogens on body parts
e.g. houseflies
• biological transmission from bites: usually involves complex life
cycle of pathogen cycling between vector and host (e.g. mosquito malaria)
Nosocomial (Hospital-Acquired) Infections
• A nosocomial infection is any infection that is acquired during the
course of stay in a hospital, nursing home, or other health care
facility.
• (2011) Healthcare-associated infections (HAI) are defined as
infections not present and without evidence of incubation at the time
of admission to a healthcare setting.
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About 5 to 15% of all hospitalized patients acquire nosocomial
infections (bloodstream infection, pneumonia, and urinary tract
infection)
Figure 14.9
1. Microorganisms in the Hospital environment
• Certain normal microbiota are often responsible for nosocomial
infections when they are introduced into the body through such
medical procedures as surgery and catheterization.
• Opportunistic, drug-resistant bacteria are the most frequent
causes of nosocomial infections.
2. Compromised Host
• Patients which are the most susceptible to nosocomial infections:
• burns
• surgical wounds
• suppressed immune systems
3. Chain of Transmission
• Chain of Transmission of nosocomial infections
• Direct contact
• staff members and patients
• between patients.
• Indirect contact
• Fomites such as catheters, syringes, and respiratory devices
• Ventilation system - airborne transmission
Control of Nosocomial Infections
• Aseptic techniques can prevent nosocomial infections.
• Washing hands before inserting a catheter
• Proper handling contaminated materials.
• Hospital infection control staff members are responsible for
overseeing the proper cleaning, storage, and handling of equipment
and supplies.
Emerging Infectious Diseases
• Diseases that are new, increasing in incidence, or showing a
potential to increase in the near future
• Contributing factors
• Genetic recombination
• E. coli O157, avian influenza (H5N1)
• Evolution of new strains
• V. cholerae O139
• Inappropriate use of antibiotics and pesticides
• Antibiotic-resistant strains
• Changes in weather patterns
• malaria
• Modern transportation
• Spread to new geographic areas
• Ecological disaster, war, and expanding human settlement
• Coccidioidomycosis
• Animal control measures
• Lyme disease
• Public health failure
• Diphtheria
Epidemiology
• The study of where and when diseases occur
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when the disease occurs
to whom (age, race, class etc.)
mode of transmission
reservoirs
effective methods of control
plans to prevent future outbreaks
• Descriptive epidemiology.
• collection of data describing the occurrence of disease (affected people, place
and period of disease)
• Analytical epidemiology.
• A group of infected people is compared with an uninfected group.
• Experimental epidemiology
• Controlled experiments designed to test hypotheses are performed.
Epidemiology
• Centers for Disease Control and Prevention (CDC)
• Collects and analyzes epidemiological information in the United States
• Publishes Morbidity and Mortality Weekly Report (MMWR)
• www.cdc.gov
• Nationally notifiable diseases (contagious and/or deadly diseases)
• Physicians are required to report occurrence of specified disease to local, state, and
national offices.
• Case reporting provides data on incidence and prevalence
• Morbidity: Incidence of a specific notifiable disease.
• Morbidity rate: Number of people affected in relation to the total population in a given time period
• Mortality: Deaths from notifiable diseases
• Mortality rate: Number of deaths from a disease in relation to the population in a given time
Epidemiology, Epidemiological graphs
Figure 14.10 - Overview
Learning objectives
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Define pathology, etiology, infection, and disease.
Define normal and transient microbiota, and microbiom.
Compare commensalism, mutualism, and parasitism, and give an example of each.
Contrast normal and transient with opportunistic microbes.
List Koch’s postulates.
Differentiate a communicable from a noncommunicable disease.
Categorize diseases according to frequency of occurrence.
Categorize diseases according to severity.
Define herd immunity.
Identify four predisposing factors for disease.
Put the following terms in proper sequence in terms of the pattern of disease: period of decline, period of
convalescence, period of illness, prodromal period, incubation period.
Define reservoir of infection.
Contrast human, animal, and nonliving reservoirs, and give one example of each.
Explain three methods of disease transmission.
Define nosocomial infections and explain their importance.
Define compromised host.
List several methods of disease transmission in hospitals.
Explain how nosocomial infections can be prevented.
Define epidemiology and describe three types of epidemiologic investigation.
Identify the function of the CDC