The human immunodeficiency virus (HIV) belongs to a family of viruses known as the "retroviruses". These viruses are known as RNA viruses because they have RNA as their basic genetic material instead of DNA. The retroviruses are unable to replicate outside of living host cells, because they contain only RNA. However, they have an enzyme called "reverse transcriptase" that can make DNA from the RNA and allow them to integrate into the host cell genome. The retroviruses are composed of three subfamilies, two of which are pathogenic to humans. They are the oncarnovirus subfamily and the lentivirus subfamily. The human immunodeficiency virus, which belongs to the lentivirus subfamily, is further divided into two types based on the diseases they produce. The HIV-1 produces the acquired iImmunodeficiency syndrome (AIDS), while the HIV-2 produces a similar disease that is restricted to West Africa.

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