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From: phil@wubios.wustl.edu (J. Philip Miller)
Newsgroups: sci.med.aids
Subject: probability of being HIV+
Message-ID: <35873@shemp.CS.UCLA.EDU>
Date: 1 Jun 90 11:50:57 GMT
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Note:	Copyright 1990 by Daniel R. Greening.  Permission granted for
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[the following was sent to me by:
From: Jimmy Dean <CSVCJLD%NNOMED.BITNET@VM.TCS.Tulane.EDU>
for posting - jpm]

     From elementary probability theory, the probability that one is
HIV+ given that one is a male homosexual is the probability that one is
a male homosexual given that one is HIV+, times the probability that one
has AIDS, divided by the probability that one is a male homosexual.

     According to the CDC, in the United States, the probability of
being a male homosexual given that one has AIDS is 2/3.  Let's assume
that the probability of being a male homosexual given that one is HIV+
is also 2/3.  Assuming 1,000,000 HIV+ people in the United States, the
probability of being HIV+ is then 1,000,000/260,000,000.  Assume that
the probability of being a male homosexual is 1/20 (half the population
is male).  Then, in the United States, the probability of being HIV+
given that one is a male homosexual must be
(2/3)*(1,000,000/260,000,000)/(1/20) = 2/39 or roughly 1/20.

     If this number is too low, then more than two thirds on the HIV+
are homosexual men, or more people are HIV+, or fewer people are male
homosexuals.  Any comments?