A big part of the reason it's such a problem in developing countries is poor nutrition. There are always exceptions to the rule, but generally speaking, strong, healthy, well-nourished bodies deal with illness differently than weak, malnourished bodies.

This is starting to become a big problem in so-called "developed" countries, too, because although we tend to eat a LOT, much what we eat doesn't really qualify as "food." It's more like "food-like product." Typical American bread, for instance, has had about 44 nutrients stripped out, then synthetic versions of about 5 put "back" in, in abnormal ratios. This "dead" flour is treated with bromine, which competes in the body for (essential) iodine, then combined with a variety of synthetic chemical ingredients before being baked. That's a far cry from anything our ancestors would have recognized as food.