Well, the answer to the question is that European settlers had a natural immunities to measles, mumps, smallpox, the plague, influenza, etc., whereas they came into contact with Indians who had never had these diseases.
Most native American deaths after contact in the 1400s was attributed to diseases.
BTW, it was a two way street. Europeans introduced syphilis, a VD which was evident only in native American societies.
But Indians were clearly the losers here in this exchange.