In 1981, homosexual men with symptoms of a disease that now are considered typical of the acquired immunodeficiency syndrome (AIDS) were first described in Los Angeles and New York. The men had an unusual type of lung infection (pneumonia) called Pneumocystis carinii pneumonia (PCP) and rare skin tumors called Kaposi's sarcoma.

The term 'hepatitis' simply means inflammation of the liver. Hepatitis may be caused by a virus or a toxin such as alcohol. Other viruses that can cause injury to liver cells include the hepatitis A and hepatitis C viruses. These viruses are not related to each other or to hepatitis B virus and differ in their structure the ways they are spread among individuals.