Clad in a red iron power suit, this American superhero is known for defending the world with the Avengers. However, Iron Man’s plates of armor proved more than just a military weapon; they also saved his life and allowed him to escape captivity to become the superhero he is today.

What distinguishes this character from his super counterparts is that unlike Spiderman and Superman, one in every 200 people in North America faces a similar condition; and they, too, can attribute their survival to these suits of iron.

Hemochromatosis is a hereditary genetic disorder that disrupts how the body metabolizes iron. A mutation in the gene HFE, which controls the amount of iron absorbed from the bloodstream, is responsible for this disease. Though rare, the condition can lead to life-threatening conditions such as cancer, heart arrhythmias, and cirrhosis. So, why would such a deadly disease be bred into the genetic code? According to Sharon Moalem, author of Survival of the Sickest, hemochromatosis is still prevalent within the population because it could have conferred a selective advantage to those suffering from the bubonic plague in the 1300s.

Normally, the body regulates the amount of iron it absorbs from food. Through releasing the peptide hormone hepcidin, it can control how much iron is absorbed by the intestines, used in body processes, and stored in various organs. Hence, when the body detects sufficient iron levels in the bloodstream, it reduces the amount of iron absorbed by the intestines, allowing any excess iron to simply pass through. In this case, if one were to stock up on iron supplements, it would still be impossible for the body to overload on iron.

Hemochromatosis disrupts the normal role of hepcidin in a person. The body consistently believes it is in a perpetual state of iron deficiency and absorbs the compound unabated, even once it has surpassed a typical absorption limit. This iron loading has deadly consequences. Over time, the excess iron is deposited throughout the body in various organs and acts as a poison by disrupting their function and causing disease. In a sense, hemochromatosis victims are rusting to death.

So, if hemochromatosis is such a lethal disease, why is it still prevalent in our population? Thanks to a process known as natural selection, everything in our genes happens for a reason.

The link between hemochromatosis and human survival can be explained by the relationship between organisms and iron. Iron plays an important role in almost every organism’s survival. Just as humans require iron for their metabolism, many forms of bacteria also depend on this compound in order to grow and propagate within their host. As a result, the human body has developed several mechanisms to sequester its iron reserves in an effort to make it unavailable to invading pathogens. Starve the pathogens, and it becomes more difficult for an infection to develop.

Curiously, people with hemochromatosis are actually less susceptible to infection than a non-hemochromatic person. Although the disease distributes excess iron throughout the body, it comes with one very beneficial side effect: the excess iron that the body absorbs isn’t distributed everywhere throughout the body. Macrophages—the police cells of the immune system—end up with significantly less iron available to pathogens than normal.

This iron-lock down in macrophages confers a significant advantage to people fighting infection. When a normal person’s macrophages encounter an invading pathogen, the pathogen may often harness the iron and use it to grow, spreading the infection throughout the lymphatic system. For a person with hemochromatosis, the lower iron content available to pathogens  in macrophages means that the cells not only have the ability to kill intruders, they can also starve the infectious agents to death.

“People who have the hemochromatosis mutation are especially resistant to infection because of their iron-starved macrophages,” Moalem explained in his novel. “So, though it will kill them decades later, they are much more likely than people without hemochromatosis to survive the plague, reproduce, and pass the mutation on to their children.”

Moalem adds, “In a population where most people don’t survive until middle age, a genetic trait that will kill you when you get there but increases your chance of arriving is, well, something to ask for.”