Southern Nevada Health District spokesman Jorge Viote said his agency is interviewing the sick and taking samples from them as part of an investigation to pinpoint the cause of the outbreak.
But he said the flu-like symptoms of vomiting and diarrhea are consistent with norovirus, a mostly food- or water-borne illness that can also be spread by an infected person.
"That's why we're advising people to wash hands and take other precautions," Viote told The Associated Press. "It can spread if people don't take precautions."
An unknown number of players and adults were taken to the hospital for treatment, he added.
Health officials were unsure how many people have fallen ill. But KSNV-TV in Las Vegas reported about 100 people have developed flu-like symptoms, including about 50 players and adults from a team in Santa Monica, Calif., and about 25 from a team in Bakersfield, Calif.
Kristiene Bulley, spokeswoman for the Bakersfield team, said players and adults began getting sick early Friday after eating Thanksgiving dinner the night before.
"We had to forfeit after the first quarter today because players were off on the side with cramps and vomit," she said Saturday. "We are headed home now to Bakersfield."
Tournament spokesman Justin Gates said at least nine of some 70 teams participating in the National Youth Football Championships have been affected. The four-day tournament ended Saturday.
He said organizers worked with the health district and the Centers for Disease Control and Prevention overnight.
"They are saying it is very likely that a player who was sick at home and came to Las Vegas on a bus likely was the cause, and it spread to that team and then to other teams," Gates told KSNV. "The best thing to do now is to manage it with proper hand-washing and following sanitary rules."
He did not immediately return phone calls to The Associated Press on Saturday.
Health officials were trying to determine the common link between those who got sick, Viote said, adding he was unsure when the health district would complete the investigation and release further details.
"It'll be hard to get a number (of sickened people) until the investigation is done," he said. "There are so many factors we don't know yet."