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2
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- Epidemiology is the study of the occurrence, distribution, and control
of disease in a population.
- Infectious diseases are illnesses caused by the presence of a pathogen
or its product.
- Acute infections occur quickly and have severe symptoms, but generally
last for a short time.
- Chronic infections have less severe symptoms over a longer time than
acute infections. Nonetheless,
they may be fatal.
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- Many pathogens can only be transmitted by direct contact.
- Pathogens, such as tuberculosis bacterium, are transmitted by droplets
in breath.
- (AIDS, hepatitis)
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- Pathogen is suspended in air and can travel a meter or more from the
host.
- Sometimes, pathogens travel in droplet nuclei, small particles that
result from evaporation of droplets.
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- A vehicle is an inanimate substance or medium which can transmit disease
to a large group of individuals.
- Food and water are common vehicles
- Inanimate objects that transmit disease are called fomites.
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- A vector is an animal that carries a disease-causing agent from one host
to another.
- Classic vectors are insects, e.g. mosquito.
- (Intermediate hosts may be included here.)
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- Sporadic occurrences are limited in scope, affecting a few individuals
in a small area, e.g., the air-conditioning system of one hotel led to
an outbreak of Legionnaire’s Disease;
- Endemic disease is caused by an infectious agent that is always present
in a population, e.g. hantavirus is endemic to rodents in SW United
States.
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- Epidemics are more widespread that endemics. For example, a high number of cases of
the flu in a single state (e.g. Michigan) would exemplify an epidemic
outbreak.
- A pandemic is a disease that occurs worldwide, e.g. the influenza
outbreak of 1918, which killed millions around the world.
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9
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- Lyme disease, first described among children in and around Lyme, CT, in
mid-1970’s was originally misdiagnosed as juvenile rheumatoid arthritis.
- Mother of one of the newly diagnosed young boys heard of several other
cases, and contacted other parents & pediatricians.
- Map revealed a cluster in location – edge of woods along Connecticut
River.
- Two moms presented cases to Dr. Allen Steere of Yale U, who saw a
cluster in time as well, and recognized similarity to tick-borne
diseases.
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10
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11
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- John Snow’s theory was remarkable, considering that it predated the
Robert Koch’s germ theory:
- “Diseases which are communicated from person to person are caused by
some material which passes from the sick to the healthy”
- Incubation is “the period which intervenes between a morbid poison
enters the system and the commencement of the illness which follows…it
is , in reality, a period of reproduction, as regards the morbid
matter”.
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- Source of infection
- drinking water contaminated with human feces
- food washed in contaminated water
- Gastric acidity is important defense
- reduces numbers of incoming bacteria
- Symptoms
- massive diarrhea, electrolyte imbalance
- net loss of 20 l of water daily
- dilute feces with mucus called "rice-water
stool"
- lesions
* hyperemia, mild inflammation
* mucosa remains intact, no invasion
- Mortality rate can reach 60% if untreated. Maintaining electrolytes is crucial.
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- Cholera epidemic of 1853 gave Snow an opportunity to test his hypothesis
and identify specific water companies supplying contaminated water.
- 300,000 individuals involved in study.
- Mortality among persons receiving water from Southwark and Vauxhall was
10x higher than Lambeth Co.
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Notes
