I take it then that no one here actually knows the failure rate of condoms with regard to protection from aids. Yet there are such fervent beliefs expressed regarding promoting condom use as "safe sex". I can not see why this posture is not utterly reckless.
If you can not understand my point regarding the nominal effect of reduction in the probability of infection per encounter upon the *final outcome* predicted by the exponential growth curve for infections, then maybe you can understand my point from an individual perspective. Surviving a string of sexual encounters without infection makes that string a set of dependent events. If q_i is the probability of surviving event i, then the probability of surviving n events is [product over all i] q_i If the probability of infection from a single encounter is p, then the probability of survival is q=(1-p), and the probability P(n) of surviving n events is: P(n) = (1-p)^n Since p is a number between 0 and 1, so is q = 1-p. Here is the important fact: [lim n -> inf] q^n = 0, when 0<q<1 This is the essence of Murphy's law (though not the many humerous but bogus correlaries.) As n goes to infinity, q^n approches zero surprisingly rapidly, even if p is small and q is close to 1. From this, given p, we can figure out how many encounters before the probability of survival is less than any given probability. Suppose we want to know how many encounters are required before the probability of infection is 90 percent. We then have: P(n) = 0.1 = (1-p)^n ln(0.1) = n * ln(1-p) n = ln(0.1)/ln(1-p) So, for example, suppose the probabilty p of infection from an encounter is 0.05. We then have n = ln(.1)/ln(0.95) = 45 At an encounter rate of 2 per week this means 90 percent probability of infection after 23 weeks. If p = 0.01, then this increases to 115 weeks, and so on. If the probability of infection in an encounter is 1/1000, then the amount of time before a 50/50 chance of infection, (ln(.5)/ln(0.999))/104, is less than 7 years. Reducing the probability of infection merely changes the amount of time before infection, not the outcome. From an aggreagate point of view, it merely changes the amount of time before some percentage of the vulnerable population is infected. Making statements that might move people from a protected group (chaste or monogamous) into an exposed group, when the probability p is not known, or if the resultant p is larger than 1/1000, is reckless and deadly. Implying the use of condoms makes for "safe sex" is such a statement. Form an aggregate point of view, condom use will have no effect on the final outcome unless a cure or vaccine is developed. It does slow the rate of progression though. It is important that the manner in which condom use is advocated does not tend to move people from a protected group into the exposed population. Barring a miracle of modern medicine, the exposed group are mostly goners. Regards, Horace Heffner
