The messages started piling up in Scott Alexander’s phone, links to studies and news articles sent by friends and family. They detailed the possible links between diabetes and dangerous complications from the novel coronavirus because of how elevated blood sugar can suppress the immune system.

Alexander, a reliever for the Los Angeles Dodgers who has diabetes, had not worried about placing himself at higher risk than any of his teammates if he returned to baseball. But he called the Dodgers’ doctors to ask for advice.

“All the conversations that I’ve had with our medical staff and our team doctors have basically said that as long as I do what I would normally do as far as keeping myself healthy, they said that I wasn’t really at any higher risk than any other person,” Alexander said. “That’s kind of been my mind-set. I trust what they’re saying. I know this is an ever-evolving situation.”

Alexander is in a class of professional athletes that adds a layer of complication and caution to the already dizzying task of sports leagues returning to play. Young and in peak physical condition, athletes are in the demographic least likely to face severe effects even if infected by the coronavirus. But among them are cancer survivors, diabetics, sufferers of autoimmune diseases and other immunocompromised players who may be at greater risk of catching the virus or more susceptible to the worst of covid-19, the disease it causes.

There may be only a handful of players who face potentially elevated health risks in each sport. But the leagues must handle them as a crucial component of return-to-play plans because the stakes for them could be literally life and death. Leagues and players’ unions also must come to an agreement on how players who might opt out for health reasons should be handled from contractual and roster standpoints. Would they be placed on injured lists? Denied salaries? Those are issues leagues have yet to sort through.

“People see professional athletes as these kind of invincible robots where they’re always in the best shape and there’s no kind of chink in their armor, but that’s not really the case,” Cleveland Browns offensive lineman and NFL Players Association president J.C. Tretter said this month in a conference call with reporters. “There are a lot of guys with underlying conditions and problems that we have to look out for. That’s something we’re always trying to keep top of mind.”

Athletes with health issues who have spoken publicly have said almost uniformly they plan to play and don’t feel endangered. Still, as potential start dates near, they will be faced with a difficult final choice.

“That would be a heavy decision,” said retired NFL linebacker Mark Herzlich, who spent seven seasons in the league after undergoing treatments to treat a rare form of bone cancer in college. “It will eventually be a heavy decision for whoever has to make that.”

Health experts say there’s not one easy answer. For starters, data on how much more at-risk immunocompromised people are to the coronavirus is still emerging. “Anybody who tells you right now, ‘I have an answer,’ beware,” Stanford infectious-disease doctor Anne Liu said. The range of preexisting conditions is huge, and even within the same condition, severity and risk differ greatly. Liu said athletes would need to take into account what sport they play, whether it’s indoors and what protocols are in place.

As one example of how clouded it can be, some experts have speculated immunocompromising medications might be protective, and some of those medications are even being tested as part of COVID-19 treatment. But Liu thinks the low percentages are resulting from those patients being more willing to take precautions to the extreme, having lived with social distancing under normal circumstances to protect themselves.

Testing will be crucial, Liu said. Right now, a reliable and fast test for a large gathering of people, such as a sports team, is not readily available.

Even the Abbott machines the White House uses have shown inconsistencies, Liu noted. But that level of testing could be available in a few months, if not sooner.