AIDS is an acronym for Acquired Immunodeficiency Syndrome, a transmissible disorder of the immune system that is caused by the human immunodeficiency virus (HIV).  HIV attacks the body's defense against disease, eventually leaving the infected individual vulnerable to malignancies and infections which cause death.  AIDS is the final stage of HIV infection, during which time these diseases arise.

The first cases of AIDS were identified in 1981 in Los Angeles, CA.  As of December 1996, there had been more than 8 million cases of AIDS worldwide, resulting in 6 million deaths.  More than 90 percent of HIV infections occur in developing countries.  Sub-Saharan Africa accounted for more than 60 percent of all infections as of 1996, in some areas almost one-third of the inhabitants were infected.

HIV is contracted mainly through exposure to blood, semen and other genital secretions, and breast milk.  Although it is a transmissable virus, it is not contagious and it can not be spread through coughing, sneezing, or casual physical contact.  The major mode of transmission worldwide -- which accounts for 70 percent of all HIV infections -- is heterosexual intercourse.  Many individuals, including a number of hemophiliacs, were infected from contaminated blood and blood products before screening procedures were introduced in the late 1980's. 

Adapted from the entry on AIDS found on brittanica.com.  Please visit brittanica.com for more information on AIDS and HIV.