From each other, for the most part. You know how it is with mortals, first one gets sick then that one shares the sickness with all their fellows; with all the various illnesses around, there's hardly a one who could tell who's healthy and who isn't.

[SO not going into the 'prevents unwanted pregnancies' aspect, not with his track record- not that they would've helped in his case anyway. That kind of prevention really doesn't have much of a track record against divinity, you know?]