A retrovirus is defined as any of a group of viruses that, unlike most other viruses and all cellular organisms, carry their genetic blueprint in the form of ribonucleic acid (RNA). Retroviruses are responsible for certain cancers and virus infections of animals and cause at least one type of human cancer. They have also been identified as the cause of acquired immune deficiency syndrome (AIDS) in humans, and they have been linked to one form of human hepatitis.

Retroviruses are so named because, by means of a special enzyme called reverse transcriptase, they use RNA to synthesize deoxyribonucleic acid (DNA). This establishes a reversal of the usual cellular process of transcription of DNA into RNA. The action of reverse transcriptase makes it possible for genetic material from a retrovirus to become permanently incorporated into the DNA of an infected cell.

The retrovirus HIV, the causal agent of AIDS, invades the target cell and infiltrates its DNA. The viral DNA is transcribed, or encoded, along with the cell's DNA into RNA, which then directs the production of proteins. The host cell doesn't recognize the viral DNA as invasive and the infection is permanent. The host continues to replicate the viral DNA and the disease spreads without impediment.