MERRIMAC - A probe into the source of what investigators believe was an outbreak of norovirus at the Hillcrest on Lake Wisconsin restaurant earlier this month is ongoing, but one official said it's possible the source of the virus may never be discovered.

Jim Kaplanek, chief of food safety for the Wisconsin Department of Health Services, said norovirus spreads so quickly that it often is difficult to determine a source. It's possible that a member of the restaurant staff spread the illness, but it also could have originated with a patron that brought it into the establishment.

"Many times all we have is a best guess, there's no conclusive evidence," Kaplanek said, adding it's difficult to determine where food-borne illnesses start. "In most cases, with food-borne illness, very rarely do you find a link between the people and the food."

Kaplanek said no food is being tested as part of the investigation, but the state is testing stool samples.

"Between 20 and 25 people were affected (by the illness) that we're aware of, and we have five people tested," said Cynthia Bodendein, a Sauk County health officer.

Kaplanek said some of the people tested positive for norovirus. Bodendein said the investigation should be complete by the end of the week.

Kaplanek said the restaurant was inspected after the outbreak was reported to the state Oct. 4, and his department recommended the restaurant implement more hand-washing and pay close attention to cleaning.

"Typically we don't shut down a facility if we find cleaning issues, we simply write orders for them to do additional cleaning," Kaplanek said. "Unless we can observe an eminent hazard to the public, we wouldn't shut down a facility."

Kaplanek said Hillcrest voluntarily closed for the day Oct. 14 and did additional cleaning and had a training session with all its employees on hand hygiene.

Kaplanek said good personal hygiene is the only prevention against norovirus, which he said is "everywhere" and causes flu-like symptoms that last 72 hours.

"It has been on the rise," Kaplanek said. "I think norovirus, if it hasn't already, is surpassing salmonella and e-coli - norovirus, is quickly going to be the number one reason for outbreaks."

Kaplanek said norovirus - commonly associated with outbreaks of illnesses on cruise ships - can be spread by employees not properly washing their hands after using the restroom and then handling food, or by a sick patron sneezing at the table.

Norma Newman, a hostess at the restaurant and the mother of the owners, said she ate the food at the restaurant the night the outbreak occurred, and she did not get sick. She said she believes the outbreak occurred because people at one table were sick, and she saw many people come up to people at that table and shake their hands.

"I ate the food, but I don't shake anybody's hands," Newman said.

In a voicemail message in the days after the incident, Thayne Newman, owner of the restaurant, said he believes the norovirus was brought into his restaurant by a customer.

"I just want to put this behind us," Thayne Newman said.