Honestly, the only answer is AIDS.
Let me explain: AIDS can only be contracted through blood. So those not infected would not get it unless they come in direct contact. Thus, when all AIDS patients die, the disease ceases (of course, some one could contract it from an animal).
I honestly believe that there is already a cure for AIDS. AIDS is not like cancer either. Once its gone from the population, its gone. If someone were to systematically integrate AIDS into a densely populated area, Earth might be able to hang on for a bit longer. Our planet is going to fall into a shit hole very soon. The United States' economy is dropping rapidly, and the Euro coin is becoming stronger (i believe), so Europe will become more powerful than America some time in the future. The problem is that Europe's population is decling and many people are emigrating from Europe. So, their economy will fal as well.
Its easy to see, for me, that the US definitely needs to switch to the Metric system (who are we kidding with this bullshit). Each small step towards a world community means greater prosperity (and most of all life) for all. This can only be is the population is controlled soon and precisely.
Listen to this song: Biohazard - Uncivilization
(of course it isnt true, but it does hold a good point)