THE CHAIN OF DISEASE TRANSMISSION
a. Each case of communicable disease is the result of an orderly progression in
a series of events. This series of events may be described as a three-link chain, each
link representing a factor essential to the transmission of disease. These links (figure 1-1)
The source of the disease (reservoir)
(2) The means by which the disease may be transmitted (mode of
A susceptible person (host).
The chain of disease transmission
b. If anyone of the links in the chain is broken, the disease cannot spread.
c. Infection is the entry and multiplication of any infectious agent in the body.
The period of time between infection and onset of signs and symptoms of disease is
called the incubation period. However, infection does not always result in recognizable
disease. Frequently the body has enough resistance or immunity to prevent disease
development, in which event a carrier state results. A carrier can transmit disease in
the same way as can a case (a person who is ill) with the same infection. Since the
carrier is infected but is not sick, the carrier often goes undiscovered as a source of
disease, or else is discovered only through very thorough clinical and laboratory effort.
When an infectious agent strikes a community or military unit, its presence quite likely
will be manifested variously in the affected individuals by all grades of severity, from the
carrier state, through the mild disease states, the typical disease states, to the most
severe and possibly fatal reactions. This broad gradation of manifestation of the
infectious process in a group is called the spectrum of infection. Within this spectrum,
there are always some cases that are so mild that they are not seen by the medical
service, while others are entirely asymptomatic. These subclinical or asymptomatic
infections, which are usually unreported, constitute what is commonly referred to as the
"iceberg effect." They, like the portion of an iceberg that is under the water, are unseen;
therefore, their danger is not fully realized by the public. In preventive medicine,