Acquired Immune Deficiency Syndrome (AIDS)  is a specific group of diseases or conditions that result from reduction in the action of the immune system  due to   infection    with     the     human immunodeficiency virus (HIV). 

In other words, HIV is the virus that causes AIDS. 

A person infected with HIV gradually loses immune function along with certain immune cells called CD4 T-lymphocytes or CD4 T-cells, causing the infected person to become vulnerable to  common ailments. With the loss of immune function, a clinical syndrome (a group of various illnesses that together characterize a disease) develops over time and eventually results in death due to opportunistic infections (infections by organisms that do not normally cause disease except in people whose immune systems have been greatly weakened) or cancers.

A person can remain HIV-positive for more than ten years without developing any of the clinical illnesses that define and constitute a diagnosis of AIDS.

HISTORY

In the early 1980s deaths by opportunistic infections, previously observed mainly in organ transplant recipients receiving therapy to suppress their immune responses, were recognized in otherwise healthy homosexual men. In 1983 French cancer specialist Luc Montagnier and scientists at the Pasteur Institute in Paris isolated what appeared to be a new human retrovirus—a special type of virus that reproduces differently from other viruses—from the lymph node of a man at risk for AIDS. At the same time, scientists working in the laboratory of American research scientist Robert Gallo at the National Cancer Institute in Bethesda, Maryland, and a group headed by American virologist Jay Levy at the University of California at San Francisco isolated a retrovirus from people with AIDS and from individuals having contact with people with AIDS. All three groups of scientists isolated what is now known as human immunodeficiency virus (HIV), the virus that causes AIDS.