A quick reminder: these are the 'each time' risks for various acts with someone who's HIV+
Q1. Receptive anal intercourse
And the answer is... somewhere between 1% and 3%. As expected, the general LJ public rated this one much higher.
There are things that can increase the risk such as having other sexually transmitted infections, for example, or if the sex leads to cuts etc. If the HIV+ person is newly infected, that also ups the risk, as they'll have more of the virus in their system before their immune system starts producing antibodies (up to three months after infection). It's these antibodies that most HIV tests look for, so they'll test HIV- at a time when they're particularly infectious...
So, when a regularly tested American male porn star became infected in 2004, he went on to infect three of the fourteen women he had anal sex with before he tested HIV+.
There are also things that can decrease the risk (beyond the obvious 'use condoms'!) such as having the insertive partner come outside.
Q2. Receptive vaginal intercourse
I expected people to know this was considerably less risky, but the public poll currently has about 60% saying it's the same or riskier.
It's about 0.2% chance, i.e. 2 in 1,000 or about 1/15th of receptive anal sex. Same caveats as above though.
Q3. Insertive anal intercourse
Now we're down to about 0.06%, i.e. 6 in 10,000 or about 1/50th of the answer to Q1. Same caveats, but it's much less risky to be the one with the penis inside.
Q4. Insertive vaginal intercourse
About the same as Q3. The range given is 0.03-0.09%, with the usual caveats.
The figure given is 0-0.04% - the high end comes from one study that looked at the behaviour of gay and bisexual men in three American cities over an extended period. By seeing who became infected and who didn't, and looking at what behaviour they reported, they came up with the 0.04% figure... but the 'confidence interval' went a little bit higher and way lower.
What's that mean? When you look at a sample of something, you don't know if you happen to have got the right figure for the whole group. It could be the sample you looked at was odd, for example. So there exist a variety of mathematical tests that tell you things like 'your answer is 0.04%, and there's a 95% chance that this is about right, and a 5% chance that it's absolute crap'. Less than 95%, and everyone goes 'it could well be crap', much more than 95%, such as 99% or the Holy Grail of 99.9% and above, and you can make a bigger fuss about what you've found.
There are other tests which say, 'ah, but given this data, any figure between 0.0001% and 0.05% would still had a 95% chance of being "about right"'... and this is what happened here. It's quite possible that 0.04%, while low, is still a big overestimate of the risk, particularly as there are other studies reckoning the risk is almost nil.
It is, however, much more socially acceptable to say 'I got it by sucking someone' than 'I got it by being fucked without a condom'.
Oral transmission can happen, but nowhere near as much as many people think. An awful lot of it goes on, nearly all of it without condoms, and the evidence suggests that's reasonable. If you want to use condoms for this, fine, if not, don't worry about the HIV risk. If he doesn't come in your mouth, the risk is as near nil as you can imagine.
(TMI disclaimer: giving oral sex is my favourite sexual activity with men, so I'd quite like to be told it's safe.)
As near nil as you can get. Even if she's menstruating.
I reckon (he says as a man!) the fuss about within the lesbian community about using dental dams for this was about an opportunity to deny the real risks - unprotected sex with men and IV drug use. It was interesting to see recently that the main users of heroin in Norway are.. young lesbians.
Q7. Sharing injecting equipment
I'll confess to having been surprised at this one - 0.67%, less than 1/100.
But I shouldn't have been - if it'd been much higher, the number of HIV+ injecting drug users would be rather higher. As it is, some people get unlucky first time, while others get away with it for longer.
In comparison, having a blood transfusion from someone who's HIV+, i.e. 'an armful' of blood straight into your bloodstream, gives over a 90% chance of infection, whereas a needle-stick injury, i.e. where you accidentally stab yourself with a needle used to inject someone who's HIV+, is 0.3%.
I'm keeping the source for the figures quiet for another day or so, because I've a few more questions to ask in another poll, but I can promise that they're well researched.
So... now you know, what are you going to do?