HIV (human immunodeficiency virus) is a virus that attacks the immune system, the body's natural defense system. Without a strong immune system, the body has trouble fighting off disease. Both the virus and the infection it causes are called HIV. White blood cells are an important part of the immune system.HIV infects and destroys certain white blood cells called CD4+ cells.ithout a strong immune system, the body has trouble fighting off disease. Both the virus and the infection it causes are called HIV. White blood cells are an important part of the immune system. HIV infects and destroys certain white blood cells called CD4+ cells.