(2) Droplet. This method of disease transmission occurs when persons are
so close together that droplets which are spread in coughing, sneezing, or even talking
are picked up by susceptible persons. Respiratory diseases are spread in this manner.
b. Indirect Transmission. Indirect transmission is the transfer of infection
without close contact between the source and the susceptible person.
(1) Arthropods. Flies, fleas, mosquitoes, and lice are among the insects
(properly called arthropods) that spread disease from man to man or from animal to
man. Such insects and related forms of life are called disease vectors. They pick up
the disease germs by coming in contact with filth or by biting an infected person or
animal. The insects may then transfer these germs to food, deposit them on a human
body, or inject them into a human body by bite.
In some instances, arthropods can be involved in direct transmission of
disease. A mosquito, for example, can pick up disease germs when it bites a
person sick with malaria. Later, the mosquito bites a susceptible person. The
mosquito is therefore the vehicle by which disease is transmitted from one
person to another.
(2) Water and food. Most of the diseases that are transmitted by water and
food result from the water or food having become contaminated by feces, urine, or other
infectious material from a case or carrier. If water or food is so contaminated and is
then not properly treated, disease agents in the material may infect the consumer.
Outbreaks of disease will occur where the practices of personal hygiene, sanitary
principles of food handling, water purification, waste disposal, and control of files and
other vermin are not properly observed and enforced. Among the intestinal diseases
usually transmitted by contaminated food or water are typhoid fever, infectious hepatitis,
dysentery, cholera, and food poisoning.
(3) Air. Some of the organisms that are expelled from the respiratory tract
of an infected individual are extremely small and light in weight. They may remain
suspended in the air for hours. If they settle out, they may be thrown back into the air
and resuspended in the dust. Inhalation of these living organisms by a susceptible
individual can result in disease. Many of the respiratory diseases are transmitted in this
(4) Fomites. Fomites are objects that may become contaminated with
disease germs from an infected individual. They may become vehicles of disease
transmission if a susceptible person uses them. Clothing, bed linen, and eating utensils
1-16. SUSCEPTIBLE INDIVIDUAL
A "susceptible" or "nonimmune" is a person who has little resistance against a
particular organism and who, if exposed to this organism, is likely to contract disease.