Acquired immune deficiency syndrome, or AIDS, is a recently recognized disease
entity. It is caused by infection with the human immunodeficiency virus (HIV),
which attacks selected cells in the immune system (see IMMUNITY) and produces
defects in function. These defects may not be apparent for years. They lead in
a relentless fashion, however, to a severe suppression of the immune system\'s
ability to resist harmful organisms. This leaves the body open to an invasion
by various infections, which are therefore called opportunistic diseases, and to
the development of unusual cancers. The virus also tends to reach certain brain
cells. This leads to so-called neuropsychiatric abnormalities, or psychological
disturbances caused by physical damage to nerve cells. Since the first AIDS
cases were reported in 1981, through mid-1992, more than 190,000 AIDS cases and
more than 152,000 deaths had been reported in the United States alone. This is
only the tip of the iceberg of HIV infection, however. It is estimated that
between 1 million and 1.5 million Americans had been infected with the virus by
the early 1990s but had not yet developed clinical symptoms. In addition,
although the vast majority of documented cases have occurred in the United
States, AIDS cases have been reported in about 162 countries worldwide. Sub-
Saharan Africa in particular appears to suffer a heavy burden of this illness.
No cure or vaccine now exists for AIDS. Many of those infected with HIV may not
even be aware that they carry and can spread the virus. It is evident that HIV
infection represents an epidemic of serious proportions. Combating it is a
major challenge to biomedical scientists and health-care providers. HIV
infection and AIDS represent one of the most pressing public policy and public
health problems worldwide.

Definition of AIDS

The U. S. CENTERS FOR DISEASE CONTROL has established criteria for defining
cases of AIDS that are based on laboratory evidence, the presence of certain
opportunistic diseases, and a range of other conditions. The opportunistic
diseases are generally the most prominent and life-threatening clinical
manifestations of AIDS. It is now recognized, however, that neuropsychiatric
manifestations of HIV infection of the brain are also common. Other
complications of HIV infection include fever, diarrhea, severe weight loss, and
swollen lymph nodes (see LYMPHATIC SYSTEM). When HIV-infected persons experience
some of the above symptoms but do not meet full criteria for AIDS, they are
given the diagnosis of AIDS-related complex, or ARC. The growing feeling is
that asymptomatic HIV infection and ARC should not be viewed as distinct
entities but, rather, as stages of an irreversible progression toward AIDS.

Historical Background

In the late 1970s, certain rare types of cancer and a variety of serious
infections were recognized to be occurring in increasing numbers of previously
healthy persons. Strikingly, these were disorders that would hardly ever
threaten persons with normally functioning immune systems. First formally
described in 1981, the syndrome was observed predominantly to be affecting
homosexual and bisexual men. Soon thereafter, intravenous drug users,
hemophiliacs, and recipients of blood transfusions were recognized as being at
increased risk for disease as well. It was also noted that sexual partners of
persons displaying the syndrome could contract the disease. Further study of
AIDS patients revealed marked depletion of certain white blood cells, called T4
lymphocytes. These cells play a crucial role in orchestrating the body\'s immune
defenses against invading organisms. It was presumed that this defect in AIDS
patients was acquired in a common manner. Then, in 1983, a T-cell lymphotropic
virus was separately discovered by Robert Gallo at the U. S. National Institutes
of Health and Luc Montagnier at France\'s Pasteur Institute. The virus was at
first given various names: human lymphotropic virus (HTLV) III,
lymphadenopathy-associated virus (LAV), and AIDS-associated retrovirus (ARV).
It is now officially called human immunodeficiency virus (HIV), and considerable
evidence demonstrates that it is indeed the causative agent for AIDS. A second
strain that has been identified, HIV-2, is thus far relatively rare outside of
Africa. Little is known about the biological and geographical origins of HIV.
Apparently, however, this is the first time in modern history that the virus has
spread widely among human beings. Related viruses have been observed in animal
populations, such as certain African monkeys, but these do not produce disease
in humans.

The Nature of the Virus

HIV is an RNA RETROVIRUS. Viewed in an electron microscope, it has a dense
cylindrical core that encases two molecules of viral RNA genetic material. A
spherical outer envelope surrounds the core. Like all retroviruses, HIV
possesses a special enzyme, called reverse transcriptase, that is able to make a
DNA copy of the viral DNA. This enables the virus