Measles

I'm a bit puzzled by numbers I'm seeing. Adult measles seem to have about 20% serious complications, and In populations with high levels of malnutrition and a lack of adequate health care, up to 10% of measles cases result in death. But the received wisdom has it that in Amerindian populations the fatality rate was higher, though perhaps this was an average over the trifecta (influenza, smallpox, measles) over a number of years, and not just a rate from one epidemic. Any ideas about resolving the apparent contradiction? Genetic susceptibility in a population not subject to many contagious diseases?