Well, when having sex with someone who has AIDS, when using a condom.. your chance of catching it is .003%... or about a 1 in 30000 chance. This number assumes vaginal sex. It increases drastically with anal.
If everybody on the planet had sex once a week with a random person, BUT USED A CONDOM... only 63000 people worldwide would be infected a year. In comparison, 63000 people die per year from the flu (in the US alone, much higher if you count the world).
The solution is to get people to use condoms. How you do this is debatable.