Naturally, the doctors did the logical thing upon discovering a superpowered mutant: They shrugged, sent the guy home, forgot all about him, and then presumably tried to drink away their crippling apathy. Other than providing an explanation as to why John was never the best swimmer (hey, Unbreakable again!), the medical world pretty much ignored what was, objectively, a medical miracle.
That is, until six years later, when another doctor found another Bruce Willis (are we going to need a plural term? Is it Bruce Willises or Bruce Willii?) who needed a hip replacement, but couldn't get one because nobody could screw the prosthesis into place. The two doctors found each other, compared notes, and realized that they had stumbled on something far more interesting than a solitary super-mutant: a secret race of super-mutants. They traced their patients' family histories and discovered that the two were related. Further investigation revealed that their entire family, scattered up and down the East Coast of the U.S., were all a bunch of indestructible supers. The world could ignore a superior form of humanity for only so long, and scientists eventually began investigating the family. Their conclusion: High bone density stems from a mutation of LRP5, the gene linked to osteoporosis and low bone density.