This section, “Infectious Diseases,” presents an
overview discussion of illness due to infection and
entries about systemic infectious diseases (illnesses
that affect the body as a whole), their treatments,
and preventive measures. Other sections in The
Facts On File Encyclopedia of Health and Medicinedis-
cuss infections specific to individual body systems.
Health, Infection, and Disease
An infection occurs when microbes—bacteria,
fungi, parasites, viruses—and other pathogens
(infectious agents) such as prions invade the body.
The infection causes illness (becomes a disease)
when it alters in some deleterious fashion the
functions of the body. Some infectious diseases
are primarily a health concern only to the people
who have them, such as NECROTIZING FASCIITIS,
TOXIC SHOCK SYNDROME, andCANDIDIASIS. These ill-
nesses are noncommunicable; they do not spread
to other people.
sIn some situations infections affect people
who have no contact with one another but some-
how share a generalized source of contamination.
These infections, such as occur with WATERBORNE
ILLNESSESin which drinking water or recreational
water contains pathogens that people consume, or
in LEGIONNAIRES’ DISEASE, in which building heating
and air-conditioning systems disperse Legionella
pneumophiliabacteria to all who breathe the build-
ing’s air, are communicable. Though contact
among infected individuals may spread the infec-
tion, the typical mode of transmission is contact
with the common source.
Numerous infections spread from one person
to another, directly such as through touching or
sharing bodily fluids or indirectly through sneez-
ing or coughing. These illnesses are not only com-
municable but also contagious: they spread easily,
rapidly, and often extensively. MEASLES, for exam-
ple, is one of the most highly contagious commu-
nicable diseases; 90 percent of people exposed to
the virus become ill with the disease. COLDS, infec-
tious mononucleosis (EPSTEIN-BARR VIRUS infec-
tion), and INFLUENZAare among the most common
contagious diseases in the United States.
Epidemics occur when large numbers of people
become ill with a communicable or contagious
disease. Throughout history these waves of infec-
tion decimated families, cities, countries, and even
entire civilizations. Smallpox, measles, bubonic
plaque, gonorrhea, syphilis, and influenza are
among the infections that raged through popula-
tions. An infectious disease is endemic when it is
always present at relatively the same rate of infec-
tion within a certain geographic region, environ-
ment, or population of people. Malaria is endemic
in Africa, for example, and consistently sickens
thousands of people.
Until the 1950s geographic boundaries con-
fined most infectious diseases, not because
pathogens (disease-causing microbes) had much
regard for natural or national borders but because
few people traveled very far from home. The
advent of commercial air flight changed all that.
By the 1970s air travel could whisk a person liter-
ally halfway around the world in less time than itINFECTIOUS DISEASES
Infectious diseases are illnesses that result from INFECTIONwith microorganisms, also called microbes. Doctors who
treat people who have infectious diseases are internists (who treat adults) or pediatricians (who treat children) who
subspecialize in infectious diseases.
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