HIV stands for human immunodeficiency virus, a virus that infects cells of the human immune system and destroys or impairs their function. Infection with this virus results in the progressive depletion of the immune system, leading to immune deficiency. Immunodeficient people are much more vulnerable to a wide range of infections, most of which are very rare among people without immune deficiency. AIDS stands for acquired immunodeficiency syndrome and describes the collection of symptoms and infections associated with acquired deficiency of the immune system. Infection with HIV has been established as the underlying cause of AIDS.