In the pre-vaccination era smallpox and measles were normal childhood diseases… and the overall childhood mortality rate was staggering by current standards. The more deadly diseases, most notably smallpox, would cause periodic epidemics. At the end of an epidemic pretty much everyone there would have had smallpox, either then or in an earlier epidemic. With nearly all immune smallpox died off locally and couldn't be spread there. Over time a new generation of susceptible children would be born, waiting for a contagious traveller to spark the next epidemic. Most that died were children, because the adults had all survived it as children. However children actually fared better than non-immune adults. They are designed to respond rapidly and strongly to any new infections. Adults fight back well against infections for which they were trained as a child, but don't respond nearly as fast or well to new ones.
When smallpox or measles was introduced to populations where no one had ever had it the death rates were catastrophic. New England's native population pre-Pilgrims, Mexico after Cortez, Hawaii and Easter Island were all but wiped out by mainly those two novel diseases. Death rates of up to 90% were reported. Today's anti-vaxed kids, who never had a natural chance to develop natural immunity through omnipresent childhood infections, if they meet up with imported measles as adults, spread through their now adult anti-vax circle of friends are going to react like Montezuma's warriors. Even if they're in perfect health their age makes them a higher risk of serious complications. Supportive care is better now; we'll save more than Montezuma's priests could, but we'll still lose some.
Like I said, the medieval time is long since past. Although I wouldn’t mind drinking cowpox out of cow milk though.