Section I. CYCLE OF INFECTION
The term "communicable disease" refers to a disease that can be easily
transmitted (communicated) from a person or animal with the disease to a person or
animal that does not have the disease. Communicable diseases may be transmitted
directly (person-to-person contact) or transmitted indirectly (something carries the
disease organism from the diseased person to the healthy person). The term
"infectious disease" is sometimes used instead of "communicable disease." In this
subcourse, both terms will be used to mean the same thing.
An infection is a condition caused by the entry and multiplication of pathogens
(disease-producing microorganisms). Pathogens of one kind or another are present in
all areas wherever life exists. They inhabit the air, soil, and water. They are also in
waste products, respiratory tracts, alimentary tracts, and on the skins of humans and
animals. Some of these organisms can survive for only a few minutes outside the
human body while others can survive for years in the environment.
THE "CYCLE OF INFECTION"
Prevention and control of infectious diseases are of vital importance to the
military. Communicable diseases are hazardous to the individual. In addition,
communicable diseases result in a great deal of lost manpower. In order for you to
understand how communicable diseases can be controlled, you should be familiar with
the components of the cycle of infection. The cycle of infection (figure 1-1) is like a
chain consisting of six links. For a communicable disease to infect a healthy person,
each link of the cycle must be present. Breaking any link in this chain can control the
spread of the disease. The six links (causative agent, reservoir, mode of escape, mode
of transfer, mode of entry, and susceptible host) are discussed in the following