I’m a survivor of the plague, how do I know this? I have a hereditary disease called hemochromatosis, inherited from eastern European descendants.

The iron-poor macrophage essentially starves the intracellular plague bacteria by not providing them with iron. This is a happy accident for us, but it isn’t as if the macrophage doesn’t already know this trick. Iron can be an important immune weapon. In mycobacterial infections (that cause pneumonia), macrophages actually raise the iron concentration in the ingested bacteria and kill them that way. In other infections, macrophages sequester their iron and starve the organisms.

Macrophage iron manipulation is not a natural immune response to Y. pestis, but HH helps to bring about the same effect, and this makes HH valuable. It is believed that many survivors of the plague in the 12th through 15th centuries had hemochromatosis. What is more, the gene is present in as many as 1/3 of living people of European descent, meaning that HH is probably massively underdiagnosed. It is likely that you know someone with HH, whether they not it or not.

Natural selection kept this mutation in the gene pool because it presented a reproductive advantage in times of plague.

biologicalexceptions.blogspot.com