This is perfect material for a third Freakonomics book: Using promiscuity to slow the spread of AIDS.

The premise is this:

1. Person A and Person B, under normal circumstances, would decide to hook up. Both are infection-free.
2. Person A decides against having sex due to STD concerns.
3. Person B still wants to find a partner.
4. Person C is not dissuaded by STD concerns, and is both available and infected.
5. Since Person A is out of the partner market, Person B sleeps with Person C instead, and becomes infected.

Had Person A decided instead to sleep with Person B, Person C would not have had a chance to infect B, and so the disease slows in its progress.

The author then goes on to discuss individual vs. group benefits, how the interest of the former can directly conflict with that of the latter… and how to work around that so that both can benefit. Great stuff.