I can't help but think that cancer is acting as a brake on the population explosion.
Cancer, in general, happens to people well past the age of reproduction. Which means it has little, if any, effect on population growth rates.
If there are diseases you'd like to keep around to prevent overpopulation, may I suggest lobbying to return Smallpox to the wild instead? Or just become a pro-AIDS activist, since the latter seems to be doing a good job of cutting into African population growth.
Seriously, some of you people scare me....