This question might be better suited from immunologists, anthropologists, etc, but I thought there might be an answer here.
Hi, the question is fine here (there are some anthropologists here, although more in /r/AskAnthropology), and in fact is asked fairly often. Not discouraging further discussion here, but fyi we have a couple of good threads on this question here
If you have follow-up questions, since the posts have been archived, just ask them here and tag the relevant user's username to notify them