It's thought to be closely related to a virus that monkeys have - earliest known human infection was back in 1959. Here's what's generally known about it:
There is a normal genetic variation on the saliva composition among humans. Some humans have no amylase activity in saliva just by genetical lottery for example
Lets suppose hypothetically than saliva would inactivate this virus in humans. Most probably there would be still some humans without this defense mechanism. And they would be totally normal humans otherwise, indistinguisable than other people unless tested chemically that just would produce a saliva with low antimicrobial activity (and could allow the disease to jump to other people).
Indeed. People have been known to get infected that way, including one I know personally. It is rare, but not unheard of.
Unrelated, but interesting: the infection rate of HIV even when having unprotected sex with somebody who is not at undetectable levels is quite low. The HIV doctor I spoke with said that it was on the order of 1 to 5 %.
> Unrelated, but interesting: the infection rate of HIV even when having unprotected sex with somebody who is not at undetectable levels is quite low. The HIV doctor I spoke with said that it was on the order of 1 to 5 %.
Male to female is more common, female to male less so. Both are rare.
> For instance, a study by Dr. Hani Miletski surveyed 93 zoophiles (82 males and 11 females). Only 12% of her sample said they engaged in sex with animals because there were no human partners available, and only 7% said it was because they were too shy to have sex with humans. For the females, the main reasons for having sex with animals was because they were sexually attracted to the animal (100%), had love and affection for the animal (67%) and/or because they said the animal wanted sex with them (67%). Most of Miletski’s sample preferred sex with dogs (87% males; 100% females) and/or horses (81% males; 73% females). Only 8% of males wanted to stop having sex with animals and none of the females. Unlike case study reports of zoophilia published prior to 2000, the studies published over the last 15 years using non-clinical samples report the vast majority of zoophiles do not appear to be suffering any significant clinical significant distress or impairment as a consequence of their behaviour.
There's extensive discussion about this in the HIV Wikipedia article. Posting such a question here seems like an odd way to learn about this topic if you're truly interested.
So where did HIV come from? Who knows. It( not the current version of HIV, something similar) has been with us for millions of years in 1 form or another. Enough to leave a large mark on what makes us, us, our DNA.
Beyond what others mentioned I recommend reading into retroviruses, in addition to the history of our understanding DNA replication and virus research. It's quite interesting that retroviruses even exist. What nature managed to accomplish with such complexity is always astonishing.
