This is a much bigger part of it than a lot of people in this thread seem to think. A lot of these diseases were extinct or close to it in the US and people have forgotten how deadly they are. The worst viral infection a lot of these people have ever seen is either a bad cold or a mild case of influenza. A good number of the younger ones have never even seen chickenpox.

It’s like people who try to pet wild animals because they’ve only ever interacted with domesticated ones and thought they were all like that.