Well, back in the 19th century, lots of children died young. My grandmother (born 1888) was one of eight. Three died as infants and one in his early 30s. Her parents were definitely not related and as far as I know, neither were her grandparents. Milk fever was probably the cause. If a woman became pregnant while she was nursing (a common occurrence) the rules were she had to quit nursing because it would harm both babies. Not true but that was the rule then among the better educated. So living on the prairie, unpasturized milk, diarrhea .... baby dies at about 18 months.
Unpasteurized milk....hell my mom (b. 1917) and the whole town drank it warm from the cow....made their own butter and constantly cooked with it. No one died.