Animation of a macrophage white blood cell infected by HIV (human immundeficiency virus), the causative agent of AIDS. The virus (not to scale) enters the cell, and causes an alteration of its cell membrane, as shown on a cell culture plate. HIV infects cells using gp120 surface proteins (green), which bind to CD4 proteins on the cell's surface, allowing the membranes to fuse and the virus contents to enter the cell.