“When you whittle down the number of MHC proteins in a population, as has happened in dairy cows bred to produce lots of milk, you wind up with a situation where viruses and bacteria can spread like wildfire.”
Think this would extend to voluntary human inbreeding? (e.g., due to ethnic/racial cleansing.) I can’t help but think of the Douglas Adams novels – my memory is a little unclear after all these years, but I think Earth eliminated the telephone sanitizers, then everyone died from a disease spread by unsanitary telephones. 
