Well if this is true then isn’t possible that ferromagnetism is the answer? Couldn’t Magneto simply be controlling the nickel inside bullets?

Beta-thalassemia is primarily a disorder of the hemoglobin protein, not iron metabolism or heme synthesis. Simply put, folks with beta-thalassemia have a mutated beta-hemoglobin protein that can’t carry oxygen well, so they receive many blood transfusions. These transfusions, from donors with many healthy, hemoglobin- and thus iron-laden red blood cells, are the source of the extra iron, for there is a greater turnover of iron when aged, transfused red blood cells are degraded in the spleen at the end of their several-month-long lifespan. It is likely that a patient with beta-thalassemia would be fairly weak anyway, and it’s not likely that Magneto would hire one for a bodyguard.

A better example of an iron-metabolism disorder would be hemochromatosis. Folks with hemochromatosis have a mutation that prevents them from disposing of the iron they take into their bodies; the iron just gets stored in macrophages in various organs. Folks with hemochromatosis might not have noticeable symptoms (arthritis, chronic fatigue, liver damage, etc) till they reach middle age, but meanwhile they may set off metal detectors at the airport. So it’s more likely that Magneto would have hired a bodyguard with hemochromatosis than beta-thalassemia.

The irony (sorry) here is that I’m basically suggesting that the X-Men discriminate against people whose mutations confer weakness rather than superpowers.