A thought experiment
http://en.wikipedia.org/wiki/Thought_experiment

During the upcoming Walk for AIDS is it possible that one person
will meet another who has HIV infection, whether they know it or
not, and have sex? Is it possible that infection could be passed
along to an uninfected person? Considering statistical
information on the spread of HIV, could it happen at least one
person after meeting another person during the AIDS walk will get
infected? Two people? Three people? Four people? More? How much
money is raised by the Walk for AIDS? How much does it cost for a
person's treatment from when HIV infection is detected until
death?

Is it possible that the AIDS Walk will result in enough new cases
of HIV and AIDS to consume all the funds generated by the event?

The strategy. Get tested together BEFORE having sex.
Get tested together for a variety of sexually transmitted
infections, including human immunodeficiency virus and share the
results with your potential sex partner BEFORE having sex
http://NotB4WeKnow.blogspot.com