In medical school, I remember being fascinated about the trade-offs that may arise when a genetic susceptibility to one disease possibly reduces the risk of another.

The example provided was how the heterozygous (unaffected) carrier of sickle cell disease, with one mutated allele, is protected against severe episodes of malaria and thus has a survival advantage in endemic areas.

However, the poor individual who inherits two copies of the mutated allele (and is therefore homozygous for sickle cell disease) is burdened by that disease.

I was not aware that there are a number of other diseases that appear to be diametrical (inversely related), such that having one disease provides an advantage against another. (Trust my brother, the evolutionary biologist, for discovering interesting material for me and my readers in the medical community.)