Death from AIDS is generally due not to HIV infection
itself, but to opportunistic infections that occur when the immune
system can no longer protect the body against agents normally found in
the environment. The appearance of any one of more than 25 different
opportunistic infections, called AIDS-defining illnesses, along with a CD4
T-cell count of less than 200 cells per cubic millimeter of blood
provides the clinical diagnosis of AIDS in HIV-infected individuals.
The most common opportunistic infection seen in AIDS is Pneumocystis
carinii pneumonia (PCP), which is caused by a fungus that normally
exists in the airways of all people. Bacterial pneumonia and tuberculosis
are also commonly associated with AIDS. In the late symptomatic phase of
AIDS, bacterial infection by Mycobacterium avium can cause fever,
weight loss, anemia,
Additional bacterial infections of the gastrointestinal tract commonly
cause diarrhea, weight loss, anorexia (loss of appetite), and fever.
Also, during advanced AIDS, diseases caused by protozoal parasites,
of the nervous system, are common.
In addition to PCP, people with AIDS often develop other
fungal infections. Thrush,
an infection of the mouth by the fungus Candida albicans, is
common in the early symptomatic phase of AIDS. Other infectious fungi
include species of the genus Cryptococcus, a major cause of meningitis
in up to 13 percent of people with AIDS. Also, infection by the fungus Histoplasma
capsulatum affects up to 10 percent of people with AIDS, causing
general weight loss, fever, and respiratory complications or severe
central nervous system complications if the infection reaches the brain.
Viral opportunistic infections, especially with members
of the herpes
virus family, are common in people with AIDS. One herpes family member,
cytomegalovirus (CMV), infects the retina of the eye
and can result in blindness. Another herpes virus, Epstein-Barr virus (EBV),
may result in a cancerous transformation of blood cells. Infections with
herpes simplex virus (HSV) types 1 and 2 are also common and result in
progressive sores around the mouth and anus.
Many people with AIDS develop cancers, the most common
types being B-cell lymphoma and Kaposi’s
sarcoma (KS). Kaposi’s
sarcoma—a cancer of blood vessels that results in purple lesions
on the skin that can spread to internal organs and cause death—occurs
mainly in homosexual and bisexual
men. Although the cause of KS is unknown, a link between KS and a new
type of herpes virus was discovered in 1994.