HIV, which causes AIDS, first starts destroying

HIV enters a person's body and enters the macrophages, where the virus' RNA genome replicates to generate viral DNA with the help of the enzyme reverse transcriptase.

This viral DNA is incorporated into the DNA of the infected cells, instructing them to create virus particles.

The macrophages continue to create viruses, and operate as an HIV factory.

HIV penetrates helper T-lymphocytes (TH), replicates and produces offspring viruses at the same time. Other helper T-lymphocytes are attacked by the progeny viruses produced in the circulation. 

This cycle is repeated, resulting in a gradual decline in the number of helper T-cells in the infected person's body.