The flu, which has reached epidemic proportions and sickened people across the country, also traveled through a good portion of the Green Bay Packers team, sidelining quarterback Aaron Rodgers for a few days last week. Earlier this month, Washington Redskins backup quarterback Kirk Cousins missed practice with the bug.
Indianapolis Colts offensive coordinator Bruce Arians was hospitalized with severe flu-like symptoms when the team played the wild-card game in Baltimore this month. San Francisco 49ers cornerback Tarell Brown played Saturday while fighting a "fever, the sickness, the flu, the whole works," his coach, Jim Harbaugh, told reporters after the team's playoff game against the Packers.
Ravens team physician Andrew Tucker said the team hasn't been hit too hard by the virus, considering the bad flu season. Only a few players have missed practice days here and there, he said.
And coach John Harbaugh didn't seem overly concerned either, even joking when asked about the flu during a news conference: "If somebody starts coughing, we immediately eject them from the meeting room," he said. "What else can we do?"
The Ravens are headed next to take on the New England Patriots near Boston, where a state of emergency was declared because of the flu outbreak. Luckily, the team is staying in nearby Providence, R.I.
The Patriots' front office said its team is fortunate that no more than a couple of players had experienced any illnesses such as the flu this season. But team personnel are knocking on wood for good luck, said a team official who didn't want to be named.
All jokes aside, because athletes are constantly in close contact when they play football, the flu has the potential to turn into bad news if not handled properly, said Tucker, who also is medical director of MedStar Union Memorial Hospital's department of sports medicine.
"It's tough once a couple of folks get it," Tucker said. "Because of the nature of the sport, with a lot of hand-to-hand contact and body-to-body contact, it can be a real challenge to prevent it from spreading."
The flu is spread by droplets that can travel as far as six feet, said Trish Perl, epidemiologist for the Johns Hopkins Health System. The football field, locker rooms and even the team plane are all ripe environments for the virus to spread.
"The team setting is much like the school setting, where you have open areas and lots of people," Perl said. "It's a risk."
But NFL teams have put in place extra precautions in recent years to protect against spreading viruses like the flu, Tucker said. The changes were prompted by panic created by elevated numbers of MRSA staph infection cases a few years ago.
At the Ravens' practice complex, hand sanitizer is positioned outside the entrance to every room and signs ask anybody with a severe cold or flu not to enter the building. Players are encouraged not to share towels and water bottles and to practice basic etiquette, like covering their mouths when they cough.
Doctors also tell the players and staff to get flu shots, although they can't force anybody to get one, Tucker said.
"We have flu shots," Harbaugh said. "We wash our hands as much as possible. We try not to cough on each other whenever possible."
Tight end Dennis Pitta got a flu shot, but said he doesn't worry too much about the virus. "You just got to do what you keep doing, eating right, staying active," he said. "That's easy for us to do. You just got to manage your health and make sure you're taking care of your body."
It's not unusual for a player to hit the field even while sick. If Redskins quarterback Robert Griffin III can play with a sprained knee, others will play with some sniffles and body aches. That is probably why Lewis played despite the flu. Lewis was spotted having a coughing fit after the win over the Broncos, Sports Illustrated reported.
"Flu?" someone asked. In between coughs, Lewis acknowledged he'd had it all week.
"Everybody takes the temperature of their own body and everybody knows what their peak is," said Carl Francis, a spokesman for the NFL Players Association.
In one legendary example of an athlete's playing through sickness, Michael Jordan dribbled his heart out in a 1997 Chicago Bulls playoff game against the Utah Jazz, despite flu-like symptoms that left him nauseous and so weak he could barely stand. He scored 38 points, getting fluids and air between quarters.
Doctors said it won't hurt a normally healthy athlete to play through sickness. If anything, it will temporarily weaken the immune system and slow down recovery, Tucker said.
"The bigger risk is if they are exposing everyone else around them," said Aaron Milstone, an assistant professor of pediatrics and an epidemiologist at Johns Hopkins.
Tucker said players are sent home if they are contagious, a state that can last as long as a week. Generally, flu patients are not contagious 24 hours after their fever breaks, Milstone said.
Doctors look at players' symptoms on a case-by-case basis, Tucker said. People with symptoms from the neck up, such as congestion and runny nose, or not as bad off as those with symptoms in the lower part of the body, such as aches and pains.
Players don't always like doctors' orders, particularly during playoff season, but it's protecting the team, Tucker said.
"We will have them stay home even though practice time and meeting time is important to them and their coaches," he said, "because the greater good sometimes is to keep them home until they're deemed safe to be around their teammates."
Baltimore Sun reporter Aaron Wilson contributed to this article.