HIV human immunodeficiency virus particles budding from a host cell. HIV is the virus that causes AIDS. Like other viruses, it enters the host cell and forces the cell磗 machinery to create more copies of the virus. The virus particles then burst out of the cell, killing it and spreading the particles through the body. HIV infects white blood cells, including CD4 T_lymphocytes and macrophages. As these are killed by the virus, the immune system becomes progressively weaker, leading the infected person to develop the symptoms of AIDS.