HIV (human immunodeficiency virus) is the virus, AIDS (Acquired Immune Deficiency Syndrome) is the syndrome caused by untreated HIV. Everybody with AIDS has HIV, but not everyone with HIV has AIDS.

The reason for the reversal in terms is treatment options. When HIV was first found, there were no treatments so AIDS was inevitable. Nowadays, medication can permanently prevent HIV from progressing to AIDS, so AIDS is much less common than HIV.