"They're calling it a norovirus," YMCA spokeswoman Monica Elenbaas said Saturday.

Noroviruses - which include Norwalk and Norwalk-like viruses - can cause diarrhea, stomach pain and vomiting for 24 to 48 hours. They are spread through food, water and close contact with infected people or things they have touched.

It was not immediately known how the campers were sickened.

To prevent the illness from spreading further, the YMCA arranged to have the campers return to Seattle on an early afternoon ferry Saturday - two days before the camp was scheduled to end.

Washington State Ferries closed the 1:30 p.m. sailing of the ferry Chelan to the general public. It arrived in Anacortes about an hour later, Elenbaas said.

The YMCA began notifying parents about the outbreak on Friday night. Many parents rode ferries over and picked their children up right away, Elenbaas said.

About 250 campers, not all of them sick, were on the Saturday afternoon ferry. Those who were sick remained inside their parents' cars throughout the trip. Those who had shown no symptoms of the illness had limited access to public areas on the ferry.

Roughly three dozen campers, including about two dozen who had fallen ill, remained at camp Saturday night. The YMCA was continuing efforts to notify their parents and make arrangements for the campers to be picked up, Elenbaas said.

The state Department of Health was conducting tests to determine the nature of the virus, Elenbaas said.

The Norwalk virus is highly contagious and has caused outbreaks of flu-like illnesses among passengers and crew on several cruise ships in recent years.