Some diseases increase the risk of developing others. Thus some conditions
increase the risk of someone developing cancer and are said to be premalignant.
This is seen in ulcerative colitis, an inflammatory condition affecting the
large intestine that increases the probability of developing bowel cancer in
sufferers. Some diseases predispose the patient to other conditions by allowing
infectious agents, not normally pathogenic, to cause disease. This is seen in
opportunistic infections where a decline in the immunological functions
of an individual makes them susceptible to infections by microorganisms
that are normally nonpathogenic. In acquired immunodeficiency syndrome
(AIDS), the individuals infected with the human immunodeficiency virus
(HIV ), (Figure 1.4) have little resistance to infections by microorganisms,
such as those responsible for causing pneumonia, as well as microorganisms
such as the yeast Candida, which are part of the normal flora of the body
(Chapter 2).
Diseases are often described as being primary orsecondary. Primary may
refer to a disease of unknown cause or idiopathic, whereas secondary is used
to refer to a condition that arises from an existing disease. However, these
terms are also often used to describe the stages of a disease. For example, in
cancer the primary tumor is the initial tumor whereas secondary tumors arise
following metastasis of the primary tumor to other tissues (Chapter 17).
Pathogenesis
Every disease has a pathogenesis that describes the development of the
disease or, more specifically, how the etiological agent(s) acts to produce
the clinical and pathological changes characteristic of that disease. Some
examples of how diseases undergo pathogenesis include inflammatory
reactions in response to harmful agents and carcinogenesis where the
formation of tumors occurs as a result of exposure to carcinogens (cancer-
inducing substances).
CHARACTERISTIC FEATURES OF DISEASESCZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY (
Figure 1.4 Electron micrograph of HIV particles
escaping from a cultured human cell. Courtesy of
Dr. D. Robertson and Professor R.A. Weiss, Institute
for Cancer Research, Royal Cancer Hospital, UK.