Sexual
The majority of HIV infections are acquired through unprotected sexual relations. Complacency about HIV plays a key role in HIV risk. Sexual transmission can occur when infected sexual secretions of one partner come into contact with the genital, oral, or rectal mucous membranes of another. In high-income countries, the risk of female-to-male transmission is 0.04% per act and male-to-female transmission is 0.08% per act. For various reasons, these rates are 4 to 10 times higher in low-income countries. The rate for receptive anal intercourse is much higher, 1.7% per act.
A 1999 meta-analysis of studies of condom use showed that the consistent use of latex condoms reduces the risk of sexual transmission of HIV by about 85%. However, spermicide may actually increase the transmission rate.
Randomized, controlled trials in which uncircumcised men were randomly assigned to be medically circumcised in sterile conditions and given counseling and other men were not circumcised have been conducted in South Africa, Kenya, and Uganda showing reductions in female-to-male sexual HIV transmission of 60%, 53%, and 51%, respectively. As a result, a panel of experts convened by WHO and the UNAIDS Secretariat has "recommended that male circumcision now be recognized as an additional important intervention to reduce the risk of heterosexually acquired HIV infection in men." Among men who have sex with men, there is insufficient evidence that male circumcision protects against HIV infection or other Sexually Transmitted Infections.
Studies of HIV among women having undergone female genital cutting (FGC) have reported mixed results, but with some evidence of increased risk of transmission. Programmes that aim to encourage sexual abstinence while also encouraging and teaching safer sex strategies for those who are sexually active can reduce short- and long-term HIV risk behaviour among young people in high-income countries, according to a 2007 Cochrane Review of studies.
Blood products
In general, if infected blood comes into contact with any open wound, HIV may be transmitted. This transmission route can account for infections in intravenous drug users, hemophiliacs, and recipients of blood transfusions (though most transfusions are checked for HIV in the developed world) and blood products. It is also of concern for persons receiving medical care in regions where there is prevalent substandard hygiene in the use of injection equipment, such as the reuse of needles in Third World countries. Health care workers such as nurses, laboratory workers, and doctors have also been infected, although this occurs more rarely. Since transmission of HIV by blood became known medical personnel are required to protect themselves from contact with blood by the use of universal precautions. People giving and receiving tattoos, piercings, and scarification procedures can also be at risk of infection.
HIV has been found at low concentrations in the saliva, tears, and urine of infected individuals, but there are no recorded cases of infection by these secretions and the potential risk of transmission is negligible. It is not possible for mosquitoes to transmit HIV.
Mother-to-child
The transmission of the virus from the mother to the child can occur in utero (during pregnancy), intrapartum (at childbirth), or via breast feeding. In the absence of treatment, the transmission rate up to birth between the mother and child is around 25%. However, where combination antiretroviral drug treatment and Cesarian section are available, this risk can be reduced to as low as one percent. Postnatal mother-to-child transmission may be largely prevented by complete avoidance of breast feeding; however, this has significant associated morbidity. Exclusive breast feeding and the provision of extended antiretroviral prophylaxis to the infant are also efficacious in avoiding transmission. UNAIDS estimate that 430,000 children were infected worldwide in 2008 (19% of all new infections), primarily by this route, and that a further 65,000 infections were averted through the provision of antiretroviral prophylaxis to HIV-positive women.
Multiple infection
Unlike some other viruses, infection with HIV does not provide immunity against additional infections, in particular, in the case of more genetically distant viruses. Both inter- and intra-clade multiple infections have been reported, and even associated with more rapid disease progression. Multiple infections are divided into two categories depending on the timing of the acquisition of the second strain. Coinfection refers to two strains that appear to have been acquired at the same time (or too close to distinguish). Reinfection (or superinfection) is infection with a second strain at a measurable time after the first. Both forms of dual infection have been reported for HIV in both acute and chronic infection around the world.
Prevention
A course of antiretroviral treatment administered immediately after exposure, referred to as post-exposure prophylaxis, reduces the risk of infection if begun as quickly as possible. In July 2010, a vaginal gel containing tenofovir, a reverse transcriptase inhibitor, was shown to reduce HIV infection rates by 39 percent in a trial conducted in South Africa. Early treatment of HIV-infected people with antiretrovirals protected 96% of partners from infection. Testing post exposure is recommended initially and at six week, three months, and six months.
There is currently no publicly available vaccine for HIV or AIDS. However, a vaccine that is a combination of two previously unsuccessful vaccine candidates (ALVAC-HIV and AIDSVAX) was reported in September 2009 to have resulted in a 30% reduction in infections in a trial conducted in Thailand. Further trials of the vaccine are ongoing.