The human immunodeficiency virus (HIV, the virus that causes AIDS) can undergo genetic changes resulting in different forms of the virus (subtypes). The existence of subtypes is a problem because the drugs that we use to treat HIV do not work on some subtypes. Subtypes vary by geographic region, with certain types being uncommon in the United States, but quite common in parts of Asia or Africa. HIV experts believe that drugs used to treat the virus may have something to do with the development of some subtypes. When the virus is exposed to a drug, it may change itself genetically so that the drug does not work anymore.