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 its 2020 season, the nation’s most prominent infectious-disease expert sounded a warning Thursday.

Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, told CNN 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 complete, on-time 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.

“Make no mistake, this is no easy task,” Allen Sills, the NFL’s chief medical officer, said in a written statement Thursday after Fauci’s comments. “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.”

The NFL plans to have teams play games in their home stadiums if that’s 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 at a single site in a bubble environment at Disney’s sports complex in Orlando.

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

“Dr. Fauci has identified the important health and safety issues we and the NFL Players Association, together with our joint medical advisers, are addressing to mitigate the health risk to players, coaches and other essential personnel,” Sills said. “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. This is based on the collective guidance of public health officials, including the White House task force, the CDC, infectious-disease experts, and other sports leagues.”

Ezekiel Elliott, the standout running back for the Dallas Cowboys, was among several players for the Cowboys and 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, confirmed that they had 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 Monday, “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 holding many meetings remotely or outdoors. But football presents unique challenges that can be difficult to address in any plan, from crowded locker rooms to larger rosters to the unavoidable physical contact of 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 gonna social distance, but we play football?” Los Angeles Rams Coach Sean McVay told reporters 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. The league and NFLPA continue to discuss those issues, and the union told agents in a conference call earlier this week that the current concept is for players and other staff members to be tested about three times per week for the virus.

“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. There is consideration being given to an earlier opening to camps so that players more time to ease into full-speed on-field activities, and to shortening the preseason from four to two games per team.

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. In the meantime, the league continues to hope for a full, on-time season, and many players have said they are taking a hope-for-the-best approach.

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