HIV infects definable population subgroups ("risk groups"). The transmission of HIV is a function of where the virus appears in the body and how it is shed. HIV can be present in a variety of body fluids and secretions, but the presence of HIV in genital secretions and in blood, and to a lesser extent breast milk, is significant for spread of HIV. However, the appearance of HIV in saliva, urine, tears, and sweat is of no major clinical importance, as transmission of HIV through these fluids does not routinely occur, primarily because of the low concentration of HIV in these fluids.
HIV is primarily spread as a sexually transmissible disease. Transmission of HIV can occur from male to male, male to female, and female to male. Female to female transmission remains extremely rare, though women with same-sex contact are often bisexual and have additional risk factors for HIV infection. The rate of HIV transmission with sexual intercourse is much lower than with other sexually transmitted diseases-approximately 0.3% per sexual contact with an HIV-infected person. However, some persons have become HIV-infected after a single sexual contact, while other persons have remained uninfected after hundreds of contacts.
Sexual contact with persons whose HIV viral load is greater, either with early infection or in the late stage of clinical AIDS, increases the transmission risk. The presence of cervical ectopia, oral contraceptive use, or pregnancy in women, intact foreskin in men, and genital ulcer disease in either sex increases the risk for HIV infection. Genital ulcers provide a more direct route to lymphatics draining to lymph nodes containing many CD4 lymphocytes and follicular dendritic cells. Tissue trauma during intercourse does not appear to play a role in HIV transmission.
HIV can be transmitted by parenteral exposure, which is the most highly efficient method of HIV transmission--close to 90%. There are many more peripheral blood mononuclear cells capable of either harboring or becoming infected by HIV in blood than are present in other body fluids or secretions. The primary risk group for HIV transmission via blood is intravenous drug users sharing infected needles. Less common practices of blood comingling or use of instruments such as tattoo needles not properly disinfected also carries a potential risk. Health care workers with percutaneous exposures to HIV-containing blood, however, are infected fewer than 1 in 300 times. Screening of blood products for HIV has almost eliminated HIV transmission by this means.
HIV infection can also be acquired as a congenital infection perinatally or in infancy. Mothers with HIV infection can pass the virus transplacentally, at the time of delivery through the birth canal, or through breast milk. Congenital AIDS occurs, on average, in about one fourth of babies born to HIV-1 infected mothers, with actual rates of transmission varying from 7 to 71%, depending upon the presence of risk factors for transmission during the course of HIV infection and pregnancy.
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