People who live in industrialized countries are physically weak when they are attacked by parasites, microbes and viruses. This is because children are increasingly being brought up in clean, sterile environments. If they are not exposed to these dangers as children they could die if "bugs" invade their bodies when they become adults.
In Ecuador, the Shuar people have a high childhood death rate, three times that in Western countries. However, as adults their immune systems are stronger. A group of Shuar adults was tested for inflammatory levels. Some had very low levels. Others had moderately high levels, below the chronic level, probably because they were in the process of becoming ill or had been sick.
Western adults have a generally high level of inflammation, constantly. There bodies fight "minor" infections all the time. Consequently, damage is caused to the body resulting in diabetes, cancer, heart attack and stroke. A Chronic level of inflammation can be a killer.
As susceptible Shuar children die, adults develop the "correct" immune response, only fighting back strongly against dangerous infections. Western adults' immune systems are not "tuned", as childhood minor infections are prevented. Basically, this is why people in developing countries may die in an epidemic, but they can survive an infective environment better than first world adults. Third world adults carry infections and have a mild immune response, when Western people would probably not survive living in such muddy conditions, particularly with the dirty water.