[NathanR]

"So, how does this really work? Thanks!"

Let P be the population of people you are talking about, say all gay males in San Francisco.

Let A be the percent of people with AIDS in this poplulation (expressed as a number between O and 1).

Let C be the condom failure rate.

Let S be the probablility that an uninfected person will get AIDS with unprotected sex with someone with AIDS.

The probability that one will get AIDS from one sex act with anyone in the population using a condom is A*C*S and the probability that one willn not get AIDS is 1 - A*C*S. Therefore, the probability that one will not get AIDS after n sex acts is (1 - A*C*S)**n (where * means multiplication and ** means to the power of).

That was probability and was the easy part. The hard part is determining what A and C and S *are*. That is statistics.

But, let us say that A is .1 or 10%, a reasonable guess in the above population, that C is .3, and that S is .1. Then A*C*S is .003 and 1 - A*C*S is .997. After 100 random sex acts, the probablity of not getting AIDS is .741, and gets worse with more sex.

[NathanR]

Using my numbers, if you have random sex once a day for a year, the probablity that you would not get AIDS in that time period is .997**365 or about .334. (using the calculator in Windows it is .997^365)

In real life, the numbers would be totally differant, and other *unknown* factors would apply.