Derek Thompson:

Stalin and Hitler: Both terrible, but also mutually destructive. Cancer and Alzheimer’s: Also both terrible and also, weirdly, mutually destructive. Apparently—and I never knew this—Alzheimer’s patients rarely have cancer. Doctors have studied the association for years without understanding the root cause. Maybe it’s mere selection effect, where people who don’t get cancer survive long enough to get dementia. Or maybe something more interesting is happening.