AIDS is the final stage of a chronic infection with the human immunodeficiency virus. There are two types of this virus: HIV-1, which is the primary cause of AIDS worldwide, and HIV-2, found mostly in West Africa. On its surface, HIV carries a protein structure that recognizes and binds only with a specific structure found on the outer surface of certain cells.
HIV attacks any cell that has this binding structure. However, white blood cells of the immune system known as T cells, which orchestrate a wide variety of disease-fighting mechanisms, are especially vulnerable to HIV attack. Particularly vulnerable are certain T cells known as CD4 cells. When HIV infects a CD4 cell, it commandeers the genetic tools within the cell to manufacture new HIV virus. The newly formed HIV virus then leaves the cell, destroying the CD4 cell in the process. No existing medical treatment can completely eradicate HIV from the body once it has integrated into human cells.
The loss of CD4 cells endangers health because these immune cells help other types of immune cells respond to invading organisms. The average healthy person has over 1,000 CD4 cells per microliter of blood. In a person infected with HIV, the virus steadily destroys CD4 cells over a period of years, diminishing the cells' protective ability and weakening the immune system (Human Immunodeficiency Virus, Cause of aids, Cause of hiv, Hiv Stage, Final Stage). When the density of CD4 cells drops to 200 cells per microliter of blood, the infected person becomes vulnerable to any of about 26 opportunistic infections and rare cancers, which take advantage of the weakened immune defenses to cause disease.
Nobody knows for sure exactly what causes HIV. Researchers certainly know how the virus is made up and how it attacks the body - but where it actually came from is a medical mystery.
That's not to say there haven't been many theories. HIV has sparked controversy and heated debate worldwide. Can we point the finger at monkeys, the polio vaccine or even a genetically engineered virus used by the CIA? Who knows?
Some of the theories are very plausible. The majority of the medical profession believes the virus was a direct descendent of the simian immunodeficiency virus (SIV). Simian means monkey.