Acquired Immunodeficiency syndrome (AIDS) is caused by the Human Immunodeficiency virus (HIV).

When HIV enters a person's bloodstream, it attacks and kills the T-helper lymphocytes, which are essential to the body in fighting off infections.

As these cells are lost, so is the body's ability to fight infection.

Possibly months after the initial infecting episode, an infected person develops a mononucleosis-like illness lasting a week or two.

A person may then be free of symptoms for years.

But as the T-helper cells die, the person becomes vulnerable to many serious infections.

The expected mortality is 100%, and there is no vaccine available to develop specific immunity.