Q: Can I get HIV from anal sex?

A: Yes, it is possible for either sex partner to become infected with HIV during anal sex. HIV can be found in the blood, cum, pre-cum, or vaginal fluid of a person infected with the virus. In general, the person being fucked or receiving the cum is at greater risk of getting HIV because the lining of the rectum is thin and may allow the virus to enter the body during anal sex. However, the person doing the fucking (or inserting his cock into an infected partner) is also at risk because HIV can enter through the urethra (the piss-slit or opening at the tip of the cock) or through small cuts, abrasions, or open sores on the cock.
Having unprotected (without a condom) anal sex is considered to be very risky behavior. If you're going to have anal sex, you can reduce the risk of HIV transmission significantly by using a latex or plastic (polyurethane) condom. Most of the time, condoms work well. However, condoms are more likely to break during anal sex than during vaginal sex. So, even with a condom, anal sex can be risky. You should also be sure to use a water-based lubricant (rather than an oil-based lube) in addition to the condom to reduce the chances of the condom breaking.
If you are barebacking, have the person fucking you pull out before cumming - or if you're doing the fucking, pull out before you cum. Avoid douching beforehand as this actually puts you at a higher risk for becoming infected with HIV. Douching may wash away helpful or "good" bacteria in the mucous membrane lining your ass as well as reducing your natural lubrication. This increases the friction of being fucked, which can cause tears in the rectum and anus. It's also not good to douche after being fucked since this could spread the virus around even more. Taking a crap afterwards may be the safer bet. Also, use lots of lube, to make sure that your asshole and rectum are as thoroughly lubricated as possible, which also helps to avoid cuts and tears which would make it easier for HIV to get into your body's blood stream.