The San Francisco 49ers shuttered the team's facilities on Wednesday after a player tested positive for Covid-19, the NFL club said in a statement.
US media reported that 49ers receiver Kendrick Bourne had returned a positive test for the coronavirus on the eve of their team's clash with the Green Bay Packers at Levi's Stadium.
"The San Francisco 49ers were informed today that a player has tested positive for COVID-19 and the individual immediately went into self-quarantine," the Niners statement said.
"Our organization has entered the NFL's intensive protocol and we are working with the league on contact tracing to identify high risk individuals. All team functions will be conducted virtually today.
"The health and safety of our players, staff and community are the organization's highest priority. We will continue to work closely with the NFL and medical professionals and will follow their direction."
The loss of Bourne is another blow to the 49ers, whose season has been disrupted by a slew of injuries.
Quarterback Jimmy Garoppolo (ankle) and tight end George Kittle (foot) both face lengthy spells on the sidelines after being injured in Sunday's defeat to the Seattle Seahawks, while receiver Deebo Samuel is also sidelined with a hamstring injury.
The Packers, meanwhile, head to California dealing with a coronavirus scare of their own.
Running back A.J. Dillon tested positive for the virus on Monday, leading to running back Jamaal Williams and linebacker Kamal Martin being withdrawn from Thursday's game in Santa Clara after they were deemed to have been in close contact with Dillon.