What we know about the NFL's first COVID-19 outbreak

In the fourth week of the regular season, the NFL is dealing with its first COVID-19 outbreak. Here's what we know so far:

• As of Sunday morning, 10 Titans players and 10 staff members have tested positive for COVID-19 since Sept. 24. Eighteen of those positive results have come since Tuesday.

• At least seven of the players -- defensive tackles Jeffrey Simmons and DaQuan Jones, long snapper Beau Brinkley, cornerback Kristian Fulton, linebacker Kamalei Correa, and receivers Adam Humphries and Cameron Batson -- were on the field during Sunday's game against the Vikings. Simmons was a defensive star in the game and played 92% of the defensive snaps. Jones played 69% of the defensive snaps and six more snaps on special teams, and Humphries and Batson played more than 50% of the offensive snaps.

• The Titans have suspended in-person team activities indefinitely. Their home game Sunday against the Steelers was postponed and rescheduled for Week 7, which had been Tennessee's bye week. Pittsburgh's original Week 7 game against Baltimore was moved to Week 8.

• Titans coach Mike Vrabel said some of those who tested positive haven't had any symptoms, while others have flu-like symptoms.

• The outbreak may have started with a member of the practice squad, defensive back Greg Maybin, who signed with the team Sept. 21 and was put on the COVID-19 reserve list Sept. 24, two days before linebackers coach Shane Bowen was kept off the trip to Minnesota because of a positive test result.

• The Vikings closed their team facility Tuesday and Wednesday, but have not had any positive test results since Sunday's game.

• The Vikings returned to practice Thursday, with stricter COVID-19 protocols and additional point-of-care testing, and their game Sunday in Houston against the Texans is going ahead as scheduled.

• Among the new protocols for exposed teams like the Vikings are: all team meetings must continue to occur virtually; gloves and masks, or face shields affixed to helmets, must be worn on the field during practices; and no team gatherings are permitted away from the facility.

• The incubation period of COVID-19 can last up to two weeks, according to the CDC, but generally appears on a test between three to five days after contact.

• Though NFL teams are not normally tested on game days, the Vikings had PCR and point-of-care tests before kickoff Sunday. On Friday, the league and its players association agreed to continue daily testing, except on game day, until further notice, and daily testing is also required during teams' bye weeks.

• The NFL, which has been fining coaches for not wearing masks properly on the sidelines, said Wednesday teams could face suspensions and the loss of draft picks for failing to comply with gameday protocols.

• The NFL and the players association have been with the Titans and are investigating whether the team violated any of the league's COVID-19 protocols.