Bolstered by a new mutation, influenza is impacting record numbers of Americans, according to U.S. CDC data. And the NFL isn’t immune.

NFL chief medical officer Allen Sills said multiple teams have been in contact with Infection Control Education for Major Sports (ICS), an independent organization established by infectious disease physicians, seeking advice on how best to curb the bug’s spread. Each team maintains its own respiratory health guidelines.

Just this past Wednesday, three New England Patriots linemen missed practice due to undisclosed illnesses (it’s unclear if they had the flu specifically). The league doesn’t track illness cases like it does on-field injuries, but before New England, it was ChicagoBaltimore, Jacksonville and Green Bay struggling to keep players healthy.

“We did the best we could as an organization in terms of trying to stop it from masks, to washing hands and being diligent that way,” Bears coach Ben Johnson said in December. “It certainly did a number on our guys.”

The ICS regularly visits team facilities to review hygiene practices and provides organizations with informational updates throughout the year. They are also focused on limiting other infectious diseases, such as staph infections.

“Those of us in medicine know that when the weather turns cold, people spend more time inside, the air is drier, you have these respiratory borne illnesses like influenza,” Sills said. “Our teams are very accustomed to that. … They all are looking for any signs of people being infected and then working to try to support them, treat them, but also isolate them from the rest of the club.”

CDC data shows that visits to the doctor for fever combined with other common flu symptoms reached their highest level nationwide since at least 1998, CNN reported, with more than 15 million people coming down with the flu this season as of Friday.

Earlier in December, Rams coach Sean McVay traveled separately from his team for a game in Arizona, reportedly to avoid getting his players sick. In past years, the Patriots have used their second team plane to isolate impacted players. This time around, the AFC No. 2 seed won’t have to travel until the AFC championship game at the earliest. And home field advantage could be more valuable than ever if it means a healthier roster.

Sills said that the COVID-era lessons around nimble outbreak responses have helped teams react to more recent flare-ups. Many franchises also installed disinfection and ventilation technology in their facilities during the pandemic that could remain useful today. 

However, COVID-19 didn’t fully shake loose the NFL’s long-held gladiator mentality. Saints linebacker Demario Davis said that, inside locker rooms, attitudes have largely returned to pre-COVID norms. There may be more guys getting flu shots, and more willingness to sit out a day of practice when under the weather, but the era of masking and distancing is over. “It’s gone back to a lot of the same ways that it probably was before,” the 14-year vet said.

Meeting with teams for recent Sunday Night Football games, analyst Cris Collinsworth said he has not noticed too many changes to the way players manage potential exposure. 

“There are always players that are really hurting,” he said. “I think there’s a little macho nature to the whole thing.”