AIDS is caused by infection with the human retroviruses HIV-1 or 2. HIV-1 is the most common cause worldwide. These viruses are passed through sexual contact; through transfusion of contaminated blood or blood products; through sharing of contaminated needles and syringes among injection drug users; intrapartum or perinatally from mother to infant; or via breast milk. There is no evidence that the virus can be passed through casual or family contact or by insects such as mosquitoes. There is a definite, though small, occupational risk of infection for health care workers and laboratory personnel who work with HIV-infected specimens. The risk of transmission of HIV from an infected health care worker to his or her pts through invasive procedures is extremely low.