The NFL must seal off players, coaches and team staff members in an enclosed community to safely play the 2020 season amid the coronavirus pandemic, Dr. Anthony Fauci, the country’s highest ranking specialist in infectious diseases, said today.

“Unless players are essentially in a bubble — insulated from the community and they are tested nearly every day — it would be very hard to see how football is able to be played this fall,” Fauci, the director of the National Institute of Allergy and Infectious Diseases, told CNN.

Fauci said if a second wave of cases sweeps the country, as many experts fear, “football may not happen this year.”

The NBA, Major League Soccer and other leagues have announced plans to create enclosed campuses where their players, coaches and staff members will live and play full time in venues without fans. The NBA, for instance, will house teams at the Walt Disney World Resort next month near Orlando, Fla., so it can finish the 2019-20 season.

The NFL has not announced any plans for constructing a similar bubble. The league is far larger than most sports, with 32 teams that each have up to 90 players during training camp, plus dozens of coaches, trainers and doctors, as well as front office staff. Unlike basketball, baseball and soccer, football requires more rest for players between games, and thus more time in isolation if a bubble was created.

The NFL acknowledged Fauci’s warning and said it would adust its guidelines to align with public health recommendations as it works to reopen training facilities. But Dr. Allen Sills, the league’s chief medical officer, offered few specifics beyond testing players.

“We are developing a comprehensive and rapid-result testing program and rigorous protocols that call for a shared responsibility from everyone inside our football ecosystem,” he said in a statement. “Make no mistake, this is no easy task. We will make adjustments as necessary to meet the public health environment as we prepare to play the 2020 season as scheduled with increased protocols and safety measures for all players, personnel and attendees. We will be flexible and adaptable in this environment to adjust to the virus as needed.”

The league and the players’ union are still trying to establish testing guidelines and other measures that would lower the risk that players get infected with the coronavirus. Training camps are scheduled to start in late July, yet players and their agents have heard few concrete measures from the union.

Even though players remain at home, several stars, including Von Miller of the Denver Broncos and Ezekiel Elliott of the Dallas Cowboys, have tested positive for the coronavirus. On Wednesday, Broncos defensive back Kareem Jackson, who has been training in Houston, also tested positive, according to NFL Network.

Dozens of college athletes, including many football players, have also tested positive for the virus since returning to their campuses for workouts.

In recent weeks, NFL coaches and team employees have returned to their offices, though under an array of conditions.

The league has said it remains committed to keeping to its schedule of opening the regular season on Sept. 10, with fans in attendance.