2-17. CAUSES OF DISEASE
There are nine major causes of disease (a through i below). Frequently a disease
may be produced by a combination of these causes, or the same disease may be
caused by different factors in different patients, or the cause may be unknown (j below).
a. Prenatal Influences. By this is meant those factors which may operate
before birth to produce disease in the offspring; factors may be manifested at birth
(congenital disease) or may not become obvious until later in life.
(1) Heredity. Among prenatal factors, one influence is heredity. A disease
may be genetically transmitted from a parent to offspring. The parents who transmit the
disease to their offspring may or may not have the disease themselves. Examples of
some hereditary diseases are hemophilia and congenital dislocation of the hip.
(2) Congenital influence. Diseases affecting the mother while she is
pregnant with the baby may adversely affect the offspring. For example, some diseases
may be transmitted directly to the baby via the bloodstream, as is often seen in the case
of syphilis in the mother. Alternatively, the pregnant woman may have a disease such
as German measles, which interferes with the normal development of the child in the
uterus (in utero), although, the child does not acquire the disease. Malnutrition in the
mother could result in a poorly nourished baby, which could also interfere with the
normal development of the child.
(3) Mechanical. Purely mechanical factors are also felt to be responsible for
some abnormalities present at birth. Abnormal positioning of the baby in utero is felt to
be occasionally responsible for wryneck; torsion or twisting of the umbilical cord would
limit the blood and food supply to the baby, and dire results could occur. Any defect or
disease present at the time of birth is called a congenital disease or condition. Injuries
or effects sustained during the process of being born may be included here.
b. Parasites. Parasites are organisms that live on or within the body of the man
or any other living organism, and at the expense of the one parasitized. Parasites may
live on the surface of the skin (ectoparasites), or they may enter the body through the
skin, the respiratory tract, the gastrointestinal tract, or the genitourinary tract where they
may enter the bloodstream and be carried to distant parts of the body. If they live inside
the body, but outside the cells, they are called extracellular endoparasites; if they enter
the body's cells, they are called intracellular endoparasites. They all cause disease by
interfering with the tissue and organ functions; they accomplish this by elaborating
toxins, or poisons; by causing inflammation, or irritation; by producing enzymes which
destroy tissue; and by causing mechanical blockage of function.
(1) Viruses. These are the smallest agents known to produce disease;
whether they are living organisms or complex chemical compounds is not known. They
are known to be intracellular endoparasites that cause such common diseases in man