Environmental Engineering Reference
In-Depth Information
a causal factor that, in concert with a necessary factor, is “sufficient” to ensure
that a disease will develop.
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A necessary factor is a type of causal factor that
is essential to, but not solely sufficient to, ensure the expression of a disease.
4
To facilitate the understanding of these factors collectively, consider the fate of
an immunocompromised person who drank contaminated tap water containing
the oocysts of
Cryptosporidium hominis
, an etiologic agent of the disease, cryp-
tosporidiosis. After about 10 days, the individual begins to express symptoms of
the disease. The afflicted person would be referred to as a case. A host factor
and, in this instance also a sufficient factor, is the immunocompromised state of
the individual. The necessary factor was the presence of the infectious material
(oocysts) in the drinking water.
Individuals most susceptible to infectious diseases, especially the illnesses
responsible to opportunistic pathogens, are the very young, the elderly, those with
cardiovascular and respiratory disease, the immunocompromised, those occupa-
tionally exposed to airborne and other pollutants, those who smoke heavily, the
obese, and those who underexercise. There are many diseases to which all persons
are considered to be generally susceptible. Among these are measles, strepto-
coccal diseases caused by group A streptococci, the common cold, ascariasis,
chickenpox, amebic dysentery, bacillary dysentery, cholera, malaria, trichinosis,
and typhoid fever. There are other diseases, such as influenza, meningococcus
meningitis, pneumonia, human brucellosis (undulant fever), and certain water-
and foodborne illnesses, to which some people apparently have an immunity or
resistance. To these should be added the noninfectious diseases such as diseases
of the heart, malignant neoplasms, and cerebrovascular diseases.
In order to reduce the number of persons who may be susceptible to a disease
at any one time, certain fundamental disease-prevention principles should be
followed to improve the general health of the public. This may be accomplished
through educational programs on personal hygiene and immunization; avoidance
of smoking; maintenance of proper weight; minimal liquor consumption; and
conserving or improving the general resistance of individuals to disease by a
balanced diet and nutritious food, fresh air, moderate exercise, sufficient sleep,
rest periods, and the avoidance of stress, fatigue, and exposure. In addition,
all individuals should be educated and motivated to protect themselves to the
extent feasible from biological, physical, chemical, and radiation hazards and
environmental pollutants.
Immunization can be carried out by the injection of vaccines, toxoids, or
other immunizing substances to prevent or lessen the severity of specific dis-
eases. Typhoid and paratyphoid fevers, poliomyelitis, and tetanus are some of
the diseases against which the armed forces are routinely immunized. Children
are generally immunized against diphtheria, tetanus, pertussis (whooping cough),
poliomyelitis, rubeola (measles), mumps, and rubella (German measles). Revac-
cination of students and others born after January 1, 1957, against measles is
recommended and may be required prior to school admission. It is now possible
to discontinue smallpox vaccination as a routine measure in view of the global
eradication of smallpox.
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