Is sexual contact the main source of HIV transmission or not?

The reason I am asking this is because I鈥檝e been doing some research online and I read some studies that say that the risk of HIV transmission through heterosexual contact is extremely low or close to zero. A few studies stated that even though HIV-positive partners engaged in sex with HIV-negative partners for a long period of time, there was no transmission of HIV. So why are there so many people with HIV, especially in places like Africa? Did they contract HIV through heterosexual contact or other risky activities such as drug use through needles? When I was growing up, all I knew and learned was that HIV is transmitted through sex. But if the risk of HIV transmission through heterosexual contact is extremely low, do people primarily get HIV through non-heterosexual contact? Please, can anybody shed some light on this?

Think of it this way, you've got people having heterosexual sex 100 billion times a year. Even if there's only a 1% chance of transmission (which it's higher than that, I'm just making up a number here), it's still going to result in more HIV infections than needle-sharing, which might have a 50% transmission rate (again just an illustrative number) but way fewer people engaging in the activity. So:
Sex --> 100 billion acts --> 1% risk = 1 billion infected
Needles --> 100 million --> 50% risk = 50 million infected

One thing you didn't mention is the use of protecton like condoms, which can limit (but not eliminate) the risk of HIV transmission. The general population of sub-Saharan Africa, Indonesia, and many other third-world countries do not have access to these kinds of protection. And it doesn't help that their corrupt governments are in full AIDS-denial mode, with some marketing homebrew teas as cures. But that's a different story. Hope this helps.

The main cause now is sexual contact, period. Haven't read those studies yet. I would really like to, so could you send them to me? Any contact with sexual fluids can cause transmission, but the more invasive the sex, the more damage to tissue, the more likely for transmission.

HIV is spread mostly through sexual contact. It is so prevalent in Africa for many reasons: Lack of information, poor health, lack of healthcare, government denial, ongoing conflicts. In Africa, HIV transmission is mostly passed heterosexually. In the US it used to be extremely homosexually passed but now more heteros are getting it. Unprotected sex is a killer.