and the rate of morbidity. The level of destruction will be so large that it will
cause a significant public health concern.
A disease becomes pandemicwhen it is distributed throughout the world. For
example, in 1918 the swine flu (influenza) reached pandemic proportions. Some
experts consider the HIV virus to be pandemic.
In a common source epidemic, large numbers of the population are suddenly
infected from the same source. These epidemics usually are attributed to a con-
taminated supply of water or improperly prepared or handled food. An example is
people who eat contaminated chicken salad at a college cafeteria. Everyone who
eats the chicken salad on this particular day will become infected and feel ill. The
epidemic will subside very fast, though, as the source of infection is eradicated.
Apropagated epidemicoccurs from person-to-person contact. The disease-
causing agent moves from a person who is infected to a person who is not
infected. In a propagated epidemic, the number of new cases rises and falls
much slower than in common source epidemics, making the pathogen much
harder to isolate and thus eliminate. An example is a “flu” virus.
Pathognomonicis a word that refers to the specific characteristics of disease.
Immunityis the specific resistance to disease. Virulenceis the degree of patho-
genicity or the capacity of an organism to produce disease.
Pathologyis the study of disease. It is derived from Pathos(“suffering”) and
logoswhich means (“science”). Pathology is that branch of discourse concerned
with the structural and functional changes that occur due to a disease-causing
agent or pathogen. A pathologist is a scientist or physician who studies the cause of
diseases, or etiology, and pathogenesis, the manner in which a disease develops.
Infection Sites
The sites where a microorganism can infect a host organism are calledreservoirs
of infection. In these sites a microorganism can maintain its ability to cause
infection. Reservoirs of infection include humans, some animals, certain non-
living media, and inanimate objects.
HUMAN RESERVOIRS
Humans make good reservoirs because they can transmit organisms to other
humans. Certain disease-causing agents have an incubation periodduring which
they are contagious and can spread the disease even before a person exhibits
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