You do realize that mass immigration has re-introduced several diseases that were wiped out on the North American continent, right? For example, polio and small pox were just bad memories for years. They're back.
Assassin bugs, native to Central and South America, have found a new home in over half of the US states thanks primarily to illegal mass immigration from those regions.
Globalization and mass movement of people create ecological time bombs all over the world. I remember some years ago a few Canadian beavers got loose in the Urals. I have no idea how they got there but man, did they ever create havoc in the area they were in.
It's more to do with the lunatics and idiots against immunization that has brought them back.