A contrary hypothesis:

Strong moral intuitions about purity do not carry significant useful knowledge about disease — and indeed can lead people to be resistant to accurate information about disease prevention. Rather, these intuitions stem from practices for maintaining group identity by refusing to share food, accommodations, or sexuality with members of rival groups. These are (memetically) selected-for because groups that do not maintain group identity cease to be groups. (This is not "group selection" — it's not that the members of these groups die out; it's that they blend in with others.)

Thus, we should expect purity memes to be strongest among people whose groups feel economically or politically threatened by foreigners, by different ethnic groups (including the threat of assimilation) or the like — and possibly weakest among world travelers, members of mixed-race or interfaith families, international traders, career diplomats, foreign correspondents, and others who benefit from engaging with foreigners or different ethnic groups.