DISEASES
Disease is any state in which the
health of the human organism is impaired. All diseases involve a breakdown of
the body's natural defense systems or those regulating the internal
environment. Even when a cause is not known, a disease can almost always be
understood in terms of the physiological or mental processes that are
disrupted.
Understanding of diseases depends on
a clear delineation of the symptoms, which are manifestations of altered life
processes. Symptoms may range from subjective reports of pain, such as headache
or backache, to visible conditions, such as a swelling or a rash. Common
symptoms include changes in body temperature (such as fever), fatigue, loss or
gain of weight, and pain or tenderness of muscles or internal organs. Further
examination may reveal: the presence of infectious organisms, as detected by
growth in special nutrient solutions; broken bones seen during radiological
examination; changes in the composition of blood cells; and cancerous growth
identified microscopically on surgically removed tissue.
With increasing use of laboratory
tests in routine physical examinations of apparently healthy people, doctors
are more often finding diseases without symptoms noticeable to the patient. For
example, high blood pressure can be detected in its early stages before it does
measurable damage to the heart and blood vessels. Another condition,
mitral-valve prolapse, in which one of the heart valves functions abnormally,
occurs in about 10% of all people examined but usually causes no symptoms.
Apparently most people with this condition are healthy, but some may have an
increased risk of autoimmune disease. The development and widespread use of
more sensitive tests may require careful use of the term disease.
Classification
Diseases can be classified by their:
site (bone, heart, liver); natural history (acute or chronic); course
(progressive or intermittent); or by other schemes. Two of the most useful ways
to classify are by cause and by which biological process is disturbed.
Classification by Cause
The classification-by-cause approach
arose from the study of infectious diseases, which were among the first
illnesses to be understood. Examples include: pneumonia, cholera and gonorrhea
caused by bacteria; smallpox, measles and hepatitis, caused by viruses. Even
when a causative organism is identified, however, infection is not the only
important part of the disease process. Many people may be exposed to the
influenza virus, for example, but some stay well, others become moderately ill
and weak or old people may die. The state of the immune system and the overall
health of the person influence the disease course.
Non-infectious causes of disease are
also known, many of them occupational. For example, long-term exposure to coal
dust predisposes miners to a respiratory disease called black lung, and
exposure to cotton dust similarly predisposes cotton workers to brown lung.
Here again, exposure to the mine or cotton dust is not the only cause of the
disease. In both cases workers who smoke cigarettes are known to have a higher
chance of developing impaired respiratory function. The same is true of the
lung disease caused by exposure to asbestos (asbestosis). More recently,
occupational exposure to fine powders and some organic chemicals has been found
to produce allergy. Scientists looking at occupational and environmental
diseases are studying why some workers exposed to these chemicals experience
breathing difficulties and severe sneezing, but others in the same work area do
not.
A new type of disease cause called
“harmful lifestyles” entered the medical vocabulary in the 1970’s. Smoking
cigarettes, drinking alcohol excessively, overeating or not exercising, and/or
living in a state of constant stress are all now considered to predispose one
to illness. These are not causes in the traditional sense. The relationship
between eating a high-fat diet and having a heart attack, for example, is even
more complicated than the process by which infection causes symptoms.
Classification by the
Process Altered
In many diseases no cause is known,
but the biological system affected can be identified. One example is cancer, in
which the normal restraint on cell growth is lost. As a result, a group of
cells anywhere in the body can grow unchecked until a large mass of cells
accumulates that can harm normal tissue. Another group of diseases results from
a derangement of the immune system. In these illnesses, called autoimmune
diseases, the immune system acts against normal tissues in the body, destroying
their normal operation.
Disease can also result from
abnormal cell receptors. Receptors are substances on the surface of cells that
allow chemicals outside the cell, such as hormones and nutrients, to enter it.
Some cases of diabetes mellitus are due to an altered cell receptor for
insulin, the hormone that promotes entry of sugar into cells. The neurological
disorder called Parkinson's disease results from an abnormal receptor for a
particular chemical necessary for brain function.
Many conditions considered to be
personality disorders are now known to have biochemical components. For
example, people with anomalies in the way they metabolize alcohol may be more
vulnerable to its effects than others. The success of some drugs in treating
mental disorders has promoted the idea that these disorders may not always be
behavioral problems, but signs of abnormal functioning of parts of the brain.
