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Chapter 12—Principles of Disease and Epidemiology.
I.
II.
III.
Pathology, Infection, and Disease.
a. Pathology: Scientific study of disease.
b. Etiology: Study of the cause of a disease.
c. Pathogenesis: Development of a disease.
d. Infection: Colonization of the body by pathogens, or the presence of a microbe in a part of the
body where it is not normally found.
e. Disease: An abnormal state in which the body is not functioning normally.
Normal Microbiota. Table 1.
a. Permanently colonize the host.
i. The human body is made up of 1 x 1013 cells (10,000,000,000,000 = 10 trillion) but it
also contains 1 x 1014 bacteria (100,000,000,000,000 = 100 trillion).
b. Normal microbiota are only found in certain regions of the body.
c. Transient microbiota may be present for days, weeks, or months.
d. Relationships Between the Normal Microbiota and the Host.
i. Microbial antagonism is competition between microbes.
1. Normal microbiota prevent harmful microorganisms from overgrowing in the
following ways:
a. Occupying niches that pathogens might occupy.
b. Producing acids.
c. Producing bacteriocins (compounds that kill other bacteria).
2. Probiotics are live microbes applied to or ingested into the body, intended to exert
a beneficial effect.
ii. Symbiosis is the relationship between a microbe and its host. Fig. 2.
1. In commensalism, one organism is benefited and the other is unaffected.
a. Many of the normal microbiota are commensals.
2. In mutualism, both organisms benefit.
a. Many of the normal microbiota are mutualists.
3. In parasitism, one organism is benefited at the expense of the other.
a. Many disease causing organisms are parasites.
e. Opportunistic Microorganisms.
i. Some normal microbiota are opportunistic pathogens.
1. They normally do not cause disease in healthy individuals.
2. They can cause disease if they colonize an area where they are not normally
found.
3. Individuals with compromised immune systems often become infected with
opportunistic pathogens.
The Etiology of Infectious Diseases.
a. Koch’s Postulates. Fig. 3.
i. Used to prove the cause of an infectious disease.
ii. Developed by Koch in 1976 when he determined the causative agent of anthrax;
postulates formalized in 1884.
1. The same pathogen must be present in every case of the disease.
2. The pathogen must be isolated from the diseased host and grown in pure culture.
3. The pathogen from the pure culture must cause the disease when it is inoculated
into a healthy, susceptible laboratory animal.
4. The pathogen must be isolated from the inoculated animal and must be shown to
be the original organism.
b. Exceptions to Koch’s Postulates.
i. Some microorganisms have unique culture requirements and cannot be cultured on
artificial media.
IV.
V.
ii. Multiple organisms can often cause the same disease signs and symptoms.
iii. Some species can cause multiple different disease conditions.
iv. Moral and ethical considerations come into play when trying to ascertain which
organisms are involved in causing diseases that only affect humans.
Classifying Infectious Diseases. [Students should read on their own]
a. Symptom: A subjective change in body function that is felt by a patient as a result of disease.
b. Sign: An objective change in a body that can be measured or observed as a result of disease.
c. Syndrome: A specific group of signs and symptoms that always accompany a disease.
d. Communicable disease: A disease that is directly or indirectly spread from one host to another.
e. Contagious disease: A disease that is easily spread from one host to another.
f. Non-communicable disease: A disease that is not transmitted from one host to another.
g. Occurrence of Disease.
i. Incidence: Fraction of a population that contracts a disease during a specific time period.
ii. Prevalence: Fraction of a population having a specific disease at a given time.
iii. Sporadic disease: A disease that occurs only occasionally in a population.
iv. Endemic disease: A disease that is constantly present in a population.
v. Epidemic disease: A disease acquired by many hosts in a given area in a short time.
vi. Pandemic disease: Worldwide epidemic.
h. Severity or Duration of a Disease.
i. Acute disease: Develops rapidly but is short-lived.
1. Influenza.
ii. Chronic disease: Develops slowly and persists for long periods.
1. Tuberculosis.
iii. Subacute disease: Intermediate between acute and chronic.
iv. Latent disease: Disease that does not display symptoms for a period of time; the disease
organism is inactive within the host during this time.
1. Shingles (varicella-zoster virus).
v. Herd immunity: When most of a population is immune to a pathogen, non-immune
individuals are less likely to come into contact with an individual that is infected.
i. Extent of Host Involvement.
i. Local infection: Pathogens limited to a small area of the body.
ii. Systemic (generalized) infection: Microorganisms or their products are spread
throughout the body via blood or lymph.
iii. Focal infection: Systemic infection that began as a local infection.
iv. Bacteremia: Bacteria in the blood.
v. Septicemia: Growth of bacteria in the blood.
vi. Toxemia: Toxins in the blood.
vii. Viremia: Viruses in the blood.
viii. Primary infection: Acute infection that causes the initial illness.
ix. Secondary infection: Opportunistic infection after a primary (predisposing) infection.
x. Sub-clinical infection: No noticeable signs or symptoms (inapparent infection).
Patterns of Disease.
a. There is normally a specific sequence of events associated with infection by a microbe and
subsequent disease:
i. A reservoir must exist.
ii. The pathogen must be transmitted to a susceptible host.
iii. The pathogen invades the host and multiplies
iv. The pathogen injures the host.
v. The illness ends when:
1. The host dies.
2. The host’s immune system destroys the pathogen.
b. Predisposing Factors.
VI.
i. Make the body more susceptible to disease.
1. Short urethra in females.
2. Inherited traits such as the sickle-cell gene.
3. Climate and weather.
4. Fatigue.
5. Age.
6. Lifestyle/nutrition.
7. Chemotherapy.
8. Radiation treatments.
c. Development of Disease. Fig. 5.
i. The specific sequence of events that occurs when a pathogen establishes itself in the host:
1. Incubation Period.
a. Time between initial infection and the appearance of signs or symptoms.
2. Prodromal Period.
a. Early mild symptoms of the disease appear; not all diseases have a
prodromal period.
3. Period of Illness.
a. The disease is most acute at this time.
b. Overt signs and symptoms appear.
c. The number of WBC’s may increase or decrease.
d. The host’s immune response usually triumphs over the pathogen to end
the period of illness. If not, the host dies.
4. Period of Decline.
a. Signs and symptoms disappear.
5. Period of Convalescence.
a. The host returns to its pre-illness state.
ii. The disease can be spread to other individuals during any of the stages above.
The Spread of Infection.
a. Reservoirs of Infection.
i. Reservoirs of infection are continual sources of infection. They can be living or nonliving.
ii. Human Reservoirs.
1. Carriers may have inapparent infections or latent infections.
iii. Animal Reservoirs.
1. Some zoonoses may be transmitted to humans. Table 2.
iv. Nonliving Reservoirs.
1. Soil, water, etc.
b. Transmission of Disease.
i. Contact Transmission. Fig. 6.
1. Direct Contact Transmission: Requires close association between an infected
individual and a susceptible host. Also known as person-to-person transmission.
a. Examples include touching, kissing, and sexual intercourse.
b. Pathogens can also be passed from animals to humans through direct
contact.
2. Indirect Contact Transmission: The pathogen is spread from its reservoir to a
susceptible host via a non-living object = fomite.
a. Tissues, cups, money, etc.
3. Droplet Transmission: Transmission via airborne droplets that travel less than
one meter from the reservoir to the new host. This is different from airborne
transmission.
a. Coughing, sneezing, talking.
VII.
VIII.
ii. Vehicle Transmission. Fig. 7.
1. Transmission by an inanimate reservoir like food, water, air, or body fluids that
are NOT fomites.
a. Waterborne Transmission: Normally a result of water being contaminated
with sewage.
b. Foodborne Transmission: Normally the result of foods being
undercooked, poorly stored/refrigerated, or handled in unsanitary ways.
c. Airborne Transmission: Transmission by airborne droplets that travel
more than one meter from the reservoir to the new host.
i. Dust, certain fungal spores, and the fine spray that accompanies
sneezes and coughs.
iii. Vectors.
1. Arthropods, especially fleas, ticks, and mosquitoes.
a. Mechanical transmission: Arthropods carry pathogens on their feet or
other body parts. Fig. 8.
b. Biological transmission: Pathogen reproduces inside the vector.
2. Table 3 lists some common Arthropod vectors and pathogens they can carry.
Nosocomial (Hospital-Acquired) Infections. Fig. 9.
a. Acquired as a result of a hospital stay.
b. 5-15% of all hospital patients acquire nosocomial infections.
i. This equates to about 2 million people per year; over 100,000 die.
c. Microorganisms in the Hospital.
i. Some normal microbiota are opportunistic pathogens.
ii. Some nosocomial pathogens are antibiotic resistant. (Table 14.4).
d. Compromised Host.
i. A host whose resistance to infection is impaired by disease, therapy, or burns.
1. Broken skin, broken mucous membranes, or a suppressed immune system are all
conditions that can compromise the host.
e. Chain of Transmission.
i. Direct contact transmission from hospital staff to patients, from patient to patient, indirect
contact transmission through fomites, and ventilation systems (airborne) are the typical
routes through which nosocomial infections are spread in hospitals.
f. Table 5 shows the common organs/systems where nosocomial infections predominantly occur in
humans.
g. Control of Nosocomial Infections.
i. Always use aseptic technique/universal precautions.
ii. Properly handle contaminated materials.
iii. Frequent and thorough hand washing with plain soap and warm water.
1. Hand sanitizers and antibacterial soaps are not recommended, unless there is no
other choice.
iv. Education of staff, patients, and visitors.
v. Appropriately use isolation rooms/wards.
vi. Make sure to thoroughly clean/disinfect rooms, sinks, tubs, toilets, etc., between patients.
vii. Prescribe antibiotics only when appropriate and necessary.
viii. Avoid invasive procedures if possible.
ix. Minimize the use of immunosuppressive drugs.
x. An infection control committee/nurse or epidemiologist should provide oversight.
Emerging Infectious Diseases.
a. Diseases that are new or changing, increasing in incidence, or showing a potential to increase in
the near future.
b. Can be caused by a virus, a bacterium, a fungus, a protozoan, a helminth, or a prion.
c. Examples are listed in Table 6.
IX.
d. Contributing factors that can lead to an EID:
i. Evolution of new strains/serovars.
1. V. cholerae O139.
ii. Inappropriate use of antibiotics and pesticides.
1. Antibiotic resistant strains. MRSA.
iii. Changes in weather patterns/global warming.
1. Hantavirus, Malaria.
iv. Modern transportation.
1. West Nile virus.
v. Ecological disaster, war, expanding human settlement.
1. Coccidioidomycosis.
vi. Animal control measures.
1. Lyme disease.
vii. Public Health failure.
1. Diphtheria. Inadequate vaccination/booster inoculation within the populace.
Epidemiology. [Students should read on their own]
a. The study of where and when diseases occur and how they are transmitted in populations.
b. Centers for Disease Control and Prevention (CDC).
i. Collects and analyzes epidemiological information in the U.S.
ii. Publishes Morbidity and Mortality Weekly Report (MMWR). Website: www.cdc.gov
1. Table 7 lists the diseases that physicians are required by law to report to the U.S.
Public Health Service; these are called “notifiable diseases.”
2. Morbidity: incidence of a specific notifiable disease.
3. Mortality: deaths from notifiable diseases.
4. Morbidity rate: number of people affected in relation to the total population in a
given time period.
5. Mortality rate: number of deaths from a disease in relation to the total population
in a given time period.