2% per event = 50% after 35 events.

Assuming 1% of partners have AIDS and someone had 10 partners and 35 events per partner.

.01 * (1 - .98)^ 35 = 0.5% per partner. And 1- (1 - .005) ^ 10 = ~5% chance of an infection in a lifetime.

Or working the other way if the average person with aids has 35 'events' per partner and 2.1 partners after infection the infection rate will increase.

PS: Real models include differing M/F infection rates, stratification by age and other vectors like transfusions and IV drug use.

IV drug use is a major vector still.

Also, HIV transmission is more likely with other STDs. If you have open syphlitic sores on your penis, you are more likely to contract HIV.

Not forgetting that HIV increases the risk of getting those other STIs too.

There's been some speculation about why rates of HIV vary so widely across Africa.

(http://www.irinnews.org/Report/91305/AFRICA-Risky-sex-does-n...)

Unsafe injection is the most efficient way to transmit the virus. It's scary that so many injections in the developing world are unsafe. (UNICEF say "16 billion injections are administered each year, of which 90 percent are for curative purposes; 50 percent of the total number of injections are unsafe.")

(http://www.irinnews.org/Report/85278/AFRICA-Poor-syringe-hyg...)

The amount of money needed to make a difference is probably (relatively) small. But changing behaviour is hard.

(http://www.path.org/our-work/safe-injection.php)

The contamination of heterosexuals man is hard. It happens, but is very hard. This information is not well spread because it can stimulate men to stop using condoms and increase the risk for women.

Without going into detail, there are sexual practices common in Africa that significantly increase the odds of transmission from woman to man.

(http://www.plusnews.org/Report/34419/ZIMBABWE-Interview-with...)

(http://www.plusnews.org/Report/31238/AFRICA-Microbicides-a-n...)

(http://www.plusnews.org/Report/41167/BOTSWANA-Bridging-the-H...)