AIDS virus particle. Coloured transmission electron micrograph (TEM) of a section through an AIDS virus particle (centre) leaving a host cell (red, lower right). The virus capsule (yellow) is covered by membrane (green) taken from the host cell. AIDS (acquired immune deficiency syndrome) is a disease caused by HIV (human immunodeficiency virus). The virus infects T-4 white blood cells, making them produce more virus particles. These erupt from the cell and infect other cells. The process kills the white blood cells. As T-4 cells are important in the immune system, the immune response is severely damaged in AIDS. This makes patients vulnerable to normally harmless diseases.