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
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
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
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.
1-10. MULTIPLE CAUSATION
The concept of multiple causation is analogous to that of the chain of disease
transmission. Just as there are three links in the chain of disease transmission, there
are also multiple causes of diseases rather than one simple cause for each disease.
The various factors of host vulnerability, agent characteristics, mode of transmission,
and reservoirs of a disease introduce a number of possibilities to which the cause of a
disease may be attributed. An exaggerated illustrative example of the multiple
causation concepts is the case of a pedestrian who was hit by an automobile and was
taken to a hospital, where he subsequently died. In the autopsy, the pathologist
attributed death to a ruptured spleen. The surgeon, however, considered the cause of
death to be slow diagnosis in the emergency room, since the spleen could have been
repaired had the diagnosis been made promptly enough. To the internist, the cause of
death was shock. The investigating policeman blamed the death on excessive speed
on the part of the driver, while the driver claimed that the pedestrian was intoxicated and
reacted improperly. Which diagnosis is correct? In all probability, there is some merit in
each of the arguments. Here we see a typical case of multiple causation where each of
the factors involved could have been the cause of death, and each contributed to the
death. Conversely, if the proper action had been taken to prevent the occurrence of
each of the events, death could have been prevented. Just as there were several
opportunities to prevent death in the example cited here, the chain of disease
transmission presents several opportunities to interrupt the continuity that is required in
the transmission of a communicable disease.