As the NFL prepares for its teams to open training camps next month and continues to express optimism about its ability to start and complete the 2020 season, the nation’s most prominent infectious-disease expert sounded a warning.

Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, told CNN in an interview released Thursday that football players would need to be placed in a “bubble” environment, isolated from others, for a season to be successfully staged this fall and winter amid the novel coronavirus pandemic.

“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 told the network. “If there is a second wave, which is certainly a possibility and which would be complicated by the predictable flu season, football may not happen this year.”

Fauci’s concern stands in contrast to the hopes repeatedly expressed by NFL leaders that they can hold a full season beginning Sept. 10. Teams are scheduled to open their training camps in late July, and the NFL continues to deliberate with the NFL Players Association over the protocols by which players are to be tested and, if necessary, treated for the virus. Allen Sills, the league’s chief medical officer, said in a phone interview Thursday evening he remains hopeful that the NFL can start its season on time and play it to completion.

“I am optimistic,” Sills said. “But I think that we all have to be realistic as well to say that none of us can predict the future. … I just think that we’re going to see a whole lot of progress that’s made, and that’s going to continue to work to our benefit.”

The NFL plans to have teams play games in their home stadiums if that is permitted under local health guidelines. That’s in contrast to the NBA, which has formulated plans to resume its season with players, coaches and other staff members gathered in a bubble environment at Disney’s ESPN Wide World of Sports Complex near Orlando.

The NFL’s plans are more like those of Major League Baseball, which would have teams playing games in their home stadiums — provided MLB is able to complete an agreement with its players’ union on economic terms to begin its stalled season.

“I greatly respect Dr. Fauci and feel like he’s raised a number of really important issues here,” Sills said. “In fact, those are the very same issues that we’ve been working on all throughout. We know that we have to construct a rigorous testing program, a rigorous screening program, surveillance program. We know we have to really carefully manage any exposure. So he has absolutely targeted the very issues that we and the NFL Players Association have been working on collaboratively.”

Sills said that he expects the league and NFLPA to have their final protocols in place “fairly soon.” He didn’t specify how much consideration the NFL and the players’ union gave — or are giving — to having all 32 teams in a single-site bubble environment, saying: “I would just say that we have considered collectively, together with the players association, a number of different models and scenarios.”

Thom Mayer, the NFLPA’s medical director, said in a written statement: “As we have communicated to our players throughout the spring, we know that there are significant challenges to the operation of football during a global pandemic. So far, we have been guided and made decisions based on the best available science and current state of infections and hospitalizations.”

Ezekiel Elliott, a standout running back for the Dallas Cowboys, was among several players from his team and the Houston Texans who reportedly tested positive for the virus recently. Others in the NFL, including New Orleans Saints Coach Sean Payton and Denver Broncos star pass rusher Von Miller, previously confirmed that they tested positive. NFL Commissioner Roger Goodell said in an interview Monday with ESPN that the positive tests of the Cowboys and Texans players would not change the league’s plans as it moves toward the opening of training camps.

“It doesn’t,” Goodell said, “because all of our medical experts indicated that as testing becomes more prevalent, we’re going to have positive tests.”

The NFL distributed an extensive set of protocols to teams for players’ eventual return to facilities, including the use of masks, physical distancing in locker rooms, and meetings occurring remotely or outdoors. But football presents unusual challenges that can be difficult to address in any plan, from crowded locker rooms to larger rosters to unavoidable physical contact during practices and games.

“I’ve seen all the memos on that, and to be quite honest with you, it’s impossible what they’re asking us to do, humanly impossible,” Baltimore Ravens Coach John Harbaugh said in a radio interview last week.

“We’re going to social distance, but we play football?” Los Angeles Rams Coach Sean McVay said during a media availability Thursday. “It’s really hard for me to understand all this. I don’t get it. I really don’t.”

The protocols do not yet include testing frequency or the steps to be taken if a player tests positive for the coronavirus. The NFLPA told agents in a conference call this week that the current concept is for players and other staff members to be tested about three times per week.

“I don’t think there is any [foolproof] protocol you can implement,” one player said Thursday, speaking on the condition of anonymity because he is involved with the NFLPA’s deliberations.

Most teams are scheduled to report to their training camps July 28. The NFL has contemplated contingencies for the regular season that include games in empty or partially filled stadiums, games being relocated or rescheduled and the season being delayed or shortened, according to people familiar with the league’s planning.

“Just pray for the players that do have it,” Washington Redskins defensive lineman Jonathan Allen said in a May video conference with reporters, “and do everything you can to take the precaution to be safe.”