HIV-1 has a very limited host range; human and chimpanzee are the only species known so far which can be chronically infected with the virus. However, no disease or deep immune depression have been observed in the hundreds of chimpanzees inoculated with human isolates of HIV-1, with one exception, whereas lack of symptoms is the exception in humans.
HIV-2 can chronically infect some macaque species (rhesus, cynomolgus); an AIDS-like syndrome has been observed in some experiments, but not in a reproducible way.
The two main routes of transmission of HIVs are blood and blood products and sexual contact. The efficiency of blood transmission (transfusion, needles, i.v. drug abuse) depends on several factors: number of virus particles, volume of blood, immune status of the receiver. Infection is particularly efficient in i.v. drug abusers.
Sexual transmission, homosexual and heterosexual, is the major mode of transmission today. All sexual practices are dangerous, but the risk is higher for anogenital intercourse, and is increased by some intercurrent genital infections (herpes, chlamydia, etc.).
Transmission from mother to child is also a major mode. In the absence of treatment, 20-30% of seropositive women give birth to an infected child. Infection can occur in the second half of pregnancy, at delivery and also by breast feeding. Severe infection of children results in death in the first year of life. Otherwise the evolution follows that seen in adults.
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