Viruses are very small particles that reproduce inside cells.

A normal cell uses DNA as a code for producing proteins, which regulate the cell's function. In order to reproduce, viruses take over the mechanism for producing proteins inside the cell, interrupting cell functions and eventually causing the cell to die.

HIV weakens the immune system, which fights infection. As part of the immune system, white blood cells help kill bacteria, viruses and fungi. The viruses enter white bloods cells, take over, and destroy the cells.

After the initial infection with HIV, a person may have no signs of AIDS for months to years, because it takes time for the virus to destroy the white blood cells. When enough of the white blood cells are destroyed, then the immune system becomes deficient: it is unable to protect the body from infection. Those who develop AIDS suffer from severe infections and cancers that are rare in those who have healthy immune systems.