Watford's weakened squad returned to training missing captain Troy Deeney and defender Adrian Mariappa.
Deeney, the Premier League club's captain, has refused to return to training in order to protect the health of his five-month-old son, who has had breathing difficulties.
Mariappa, a 33-year-old defender, was one of three positive tests at Watford with the other two among staff members.
The Hornets accounted for half the six Covid-19 cases detected from 748 tests of Premier League players and staff ahead of a return to training in small groups this week.
"Ever since I got my positive result back on Tuesday, I've been scratching my head to try to work out how I might have got coronavirus," Mariappa said.
"It was a big surprise because I haven't really left the house, apart from some exercise and the odd walk with the kids. I've mainly just been homeschooling and keeping fit."
Other Watford players are reportedly set to follow Deeney's example, but Mariappa did not rule out returning to training once he was gone through a period of seven days self-isolation.
"I will just follow the protocols and each of the steps, and decide what is right for me each time," he added.
"My focus now is just on isolating for seven days and hopefully testing negative next time."
Watford sit in 17th position in the Premier League, only outside the relegation zone on goal difference.