The human immunodeficiency virus (HIV) is a retrovirus, which, like many other viruses, stores its genetic information as RNA rather than as DNA (most other living things use DNA).

When HIV enters a human cell, it releases its RNA, and an enzyme called reverse transcriptase makes a DNA copy of the HIV RNA. The resulting HIV DNA is integrated into the infected cell’s DNA. This process is the reverse of that used by human cells, which make an RNA copy of DNA. Thus, HIV is called a retrovirus, referring to the reversed (backward) process.

Other RNA viruses (such as polio, influenza, or measles), unlike retroviruses, do not make DNA copies after they invade cells. They simply make RNA copies of their original RNA.

Each time an HIV-infected cell divides, it makes a new copy of the integrated HIV DNA as well as its own genes. The HIV DNA copy is either

• Inactive (latent): The virus is present but does no damage.

• Activated: The virus takes over the functions of the infected cell, causing it to produce and release many new copies of HIV, which then invade other cells.