A malady formerly attached to “punch drunk” boxers, C.T.E. has now been found posthumously in more than 200 former football players, including more than 100 who played in the N.F.L., plus an array of athletes in sports ranging from hockey to soccer, rodeo to BMX. Much of the research about the condition has centered on brain injuries in the military.

Scientists are on the verge of being able to confidently diagnose C.T.E. in the living. It promises to be a game-changer, leading to all sorts of complex ethical questions in sports.

When a football star receives a C.T.E. diagnosis, for example, who will decide whether he should stop playing? Will high schools, colleges and professional teams have an obligation to test and reveal the results? Will athletes in all sports, at all ages, have the option to be tested?

Hairston embodied some of that coming anxiety. During news media interviews in recent years, he sometimes mentioned offhandedly that he thought he had C.T.E. The conversations always moved on quickly.

Even those who hunted with him regularly said they did not see cause for concern. Paul Bride, an outdoors photographer who worked for KUIU, accompanied Hairston on all his expeditions.

“I saw him on his best days, in the best place he could be — the mountains,” Bride said.

In hindsight, though, his friends said maybe Hairston had become more forgetful, a bit less predictable. None of those close to him were alarmed enough to worry that he might end his life. He had it all.

Privately, though, the Hairstons struggled to hold it together. Jason Hairston routinely broke down and cried, Kirstyn said, scared of where his brain was headed. When a scan revealed deterioration in the frontal lobe that had not been present a year earlier, she said, Hairston made her promise she would never make him have another test, because he did not want to know the results.