Figure 1-2. Break in the chain of disease transmission.
1-14. SOURCE OF DISEASE (RESERVOIR)
The source of disease (infection) may be a case, a carrier, or an animal.
a. Case. A person who is actually ill with a disease is called a case. A case is a
common source of infection.
b. Carrier. A person who harbors disease organisms, but who is not ill is called
a carrier. This person can spread the germs in the same manner as does the case and
is actually more dangerous, because he may not know that he is harboring the
c. Animals. Animals, especially rodents, are reservoirs for certain diseases that
also affect humans. The animal reservoirs may be sick, or they may harbor the disease
in much the same way as a human carrier. The term animal means any member of the
animal kingdom, thus including insects as well as mammals.
1-15. METHODS OF TRANSMISSION (VEHICLES)
Diseases may be transmitted either by direct or indirect methods.
a. Direct Transmission. Direct transmission is a method of transmission in
which the infectious organisms pass directly from one person to another. It may happen
in either of the following ways:
(1) Physical contact. Certain diseases are spread by physical contact with
an infected individual. Examples are syphilis, gonorrhea, and scabies.