The NFL Players Association advised players Saturday not to participate in group workouts ahead of the scheduled opening of teams’ training camps late next month, based on concerns related to the novel coronavirus pandemic.

“Please be advised that it is our consensus medical opinion that in light of the increase in Covid-19 cases in certain states that no players should be engaged in practicing together in private workouts,” the union’s medical director, Thom Mayer, wrote to all NFL players. “Our goal is to have all players and your families as healthy as possible in the coming months.”

The NFL has joined Major League Baseball, the NHL and college programs in having a recent flurry of players test positive. That comes as some states nationwide experience an increase in cases while lifting restrictions and reopening businesses.

“We are working on the best mitigation procedures at team facilities for both training camps and the upcoming season, and believe that it is in the best interest of all players that we advise against any voluntary joint practices before training camp commences,” Mayer wrote in Saturday’s message, which was posted to the NFLPA’s Twitter account.

The NFLPA is referring to informal workouts being conducted by players on their own, not under teams’ supervision or subject to testing protocols. Only players receiving medical treatment or rehabilitating from injuries are permitted in teams’ facilities at this point.

There have been reports of recent positive tests involving players from the Dallas Cowboys, Houston Texans, San Francisco 49ers and Tampa Bay Buccaneers. Ezekiel Elliott, the standout running back for the Cowboys, was among the players reported to have tested positive. An unidentified 49ers player who was working out in Nashville with teammates was reported to have tested positive.

The Buccaneers confirmed in a statement Saturday that individuals had tested positive at their training facility, without providing specifics. The Buccaneers said those who may have been exposed to the virus have been notified and are self-isolating, and the affected areas of the facility have been closed for sanitizing. ESPN reported at least two players were among those in the Buccaneers organization who had tested positive.

NFL Commissioner Roger Goodell and Allen Sills, the league’s chief medical officer, have said the league expects to have to deal with positive tests for players and others as it moves forward with its plans to play the 2020 season. Sills said in a phone interview Thursday evening that he is being realistic about the issues the league is facing but remains optimistic that the NFL can start its season on time in September and play it to its completion.

“I think we just have to remain in very close contact with public health authorities and with our scientific advisers to look at what’s safest in all of our locations,” Sills said.

Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, told CNN earlier Thursday that “football may not happen this year” unless players are isolated from others in a “bubble” environment.

Sills declined to specify during Thursday evening’s phone interview how many players testing positive would require a team to be shut down during training camp or the season.

“I think that’s a topic that we’re still actively discussing and thinking about how we can best approach,” Sills said.

NFL teams have conducted their offseason programs for players virtually, with no on-field practices. Those programs must end by June 26. The NFL reopened teams’ facilities to coaches and other staffers and has sent protocols to teams for the eventual return of players. Those protocols do not yet include testing and treatment details, which remain under discussion between the league and players’ union. The NFLPA told agents Monday that it expects players to be tested about three times per week. Most NFL teams are scheduled to open their training camps July 28.