Europeans brought measles with them when they arrived in America. It was already a childhood disease then. In America, among Europeans, it has never been a big killer. It probably killed a lot of the native population, but there were many "new" diseases doing that.
Measles then and now is a killer
Its been around for thousands of years.
Between roughly 1855 and 2005, measles has been estimated to have killed about 200 million people worldwide.
2018 world wide close to 150,000 died
In the US vaccination did not start until the early sixties