The full form of HIV is human immunodeficiency virus. It belongs to genus lentivirus. It is different from the other retro viruses. This virus attacks and destroys the immune system.in our body white blood cells plays a very important role in immune system, this virus destroy the white blood cells and damage the immune system. Large number of white blood cells are destroyed we lose our resistance power our body not defend against infection. The HIV virus mainly transferred by the blood and unprotected sex.