HIV causes AIDS. Two HIV strains have been identified: HIV-1 and HIV-2. HIV-1 is the prototype virus and is responsible for most cases of AIDS in the United States. HIV-2 is found chiefly in West Africa, appears to be less easily transmitted, and has a longer incubation period. Susceptibility to infection is unclear. The presence of sexually transmitted infections (STIs) with open lesions, such as herpes and syphilis, may increase the patient's susceptibility to viral entry. People with cytomegalovirus and Epstein-Barr virus infections may also be more susceptible because of an increased number of target cells. Routes of transmission are through sexual contact (male to male, male to female, female to male, and female to female); by blood to blood or transfusion contact (generally blood products given between 1977 and 1985); through the use of needles contaminated by an HIV-infected person; by blood or other HIV-infected fluids coming in contact with open lesions or mucous membranes; and by mother to child during the in utero period, during delivery, or by breastfeeding. The time from the onset of HIV transmission to the development of AIDS varies from a few months to years. The median incubation period is 10 years. High-risk behaviors that are linked to HIV infection include unprotected anal and vaginal intercourse, having multiple sex partners, and using nonsterile drug injection equipment.

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