Lesson 1: Introduction to Disease Transmission and Epidemiology

1-9

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1-9. SUSCEPTIBLE PERSON (HOST)

A susceptible or non-immune person is one who has little resistance against a particular organism and who, if exposed to this organism, is likely to contract disease. By contrast, an immune person is one who has a high degree of resistance to the organism and who, when exposed, does not develop the disease. Immunity to many diseases is relative and can be overcome by sufficient exposure to the diseases. The term host is used to connote the living body upon which or in which a disease agent or parasite lives--the final recipient of an infectious agent. The susceptibility of a host to invasion by infectious agents is dependent upon a number of factors.

a. Age. The very young and the very old are usually more susceptible to diseases than older children and young adults. In addition, certain diseases have a natural affinity for persons in certain age groups. Examples of these diseases are measles, chickenpox, mumps, and other diseases normally associated with childhood and adolescence.

b. Physical Condition. Persons who are in a state of malnutrition, suffering from extreme fatigue or exposure to the elements, or suffering from an imbalance in normal body functions have weakened resistance to the invasion of pathogenic organisms.

c. Immunity. The natural immunity acquired by an individual by virtue of having been infected by, or exposed to, an infectious organism and having developed antibodies against the agent reduces the likelihood of further or repeated infection. Similarly, one who has been artificially immunized against a disease will be similarly protected.

d. Habits and Customs. Local mores--particularly those found in undeveloped and underdeveloped regions--may be the cause of increased susceptibility to disease. Although these conditions are more appropriately discussed under sanitary practices than as host factors, their longstanding acceptance by certain cultures makes them inseparable from the host. Examples of these practices are the custom of rubbing the umbilical cord with dirt (in some South American cultures) and of defecating directly into rice paddies (as in Vietnam and other Asian cultures).

e. Other Factors. A number of other factors tend to predispose an individual to various diseases--not necessarily communicable diseases, but infirmities in general. Among these factors are race (hypertensive heart disease and sickle cell anemia in blacks); sex (breast cancer in women); occupation (lung cancer in asbestos workers); geographical location; socioeconomic status; and others.

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