Wait, so the vast pre-Columbus North American population lived in giant crowded permanent cities... that left no archeological record? What?????

The problem with this theory isn't the hypothetical virulence of smallpox, the problem is that it's demonstrably idiotic. Huge settlements don't vanish without a trace- no trace means no huge settlements.

But the lactose intolerance thing is true. Different populations have different alleles, and most people in the world who aren't of Northern European, Middle Eastern, Mongolian, Fulani or Masai descent decrease their lactase production in adulthood to the point where it becomes difficult to metabolize large quantities of milk. This isn't a racist idea any more than the idea that there's a higher prevalence of sickle cell anemia in people of East African descent.

~ Another Mouse