Actually it's your proposed comparison that's useless. If you and your neighbor are both exposed to the same bad environmental conditions and you get sick and y

Actually it's your proposed comparison that's useless. If you and your neighbor are both exposed to the same bad environmental conditions and you get sick and your neighbor doesn't, that's probably a matter of luck in the genetic lottery, which you can't do anything about. To isolate the environmental conditions - stuff you can do something about - you have to compare a population exposed to one environment with another population exposed to another environment, which is what they are doing.