(AP) -- U.S. health officials say last fall's outbreak of a stomach virus that swept through pro basketball teams sickened 21 players on 13 teams.
They were infected with the norovirus, highly contagious and known for spreading on cruise ships. The federal researchers didn't name the teams. But media reports have said players with a stomach bug at the time included four on the Orlando Magic, including star center Dwight Howard.
The virus can spread through the air on the basketball court. But researchers said it more likely was spread by players socializing.
The Centers for Disease Control and Prevention initiated the investigation after reading media reports about player illnesses, and the NBA cooperated, said Dr. Rishi Desai, one of the CDC investigators.
It's not clear how the outbreak started, but the strain of norovirus was an unusual enough that investigators believe players got it from each other rather than from family members or others, he added.
CDC officials believe that there may have been at least two instances in which an infected team passed it to a healthy team. The CDC recommended that in the future, NBA players sick with the virus should not play or take part in team activities.
Norovirus can cause diarrhea, vomiting and stomach pain. It often spreads through contaminated food or water, but the virus can also move through dried vomit particles spread in the air, Desai said.
An NBA spokesman could not immediately be reached for comment.
The results of the investigation were publicly disclosed for the first time Friday at a CDC conference in Atlanta.
Explore further: S. African disease lab on frontline of Ebola fight