Story highlights At least 135 people sickened in one outbreak linked to a restaurant

Health officials identify the outbreak as Norovirus

"We are not sure yet how it started," Harbor Inn Seafood manager says

A series of viral outbreaks causing diarrhea and vomiting has affected scores of people in North Carolina in recent weeks, sickening them, but causing no fatalities, health officials there said Wednesday.

Several health departments have reported multiple outbreaks, prompting officials to urge residents to take precautions to prevent possible exposure to the virus, which they identified as norovirus.

The largest outbreak, with 135 cases traced to a single restaurant, was found in Catawba County, said Kelly Schermerhorn, a public information officer with the Catawba County Public Health Department. She said the cases were linked to the Harbor Inn Seafood Diner in Conover. The restaurant is about an hour northwest of Charlotte.

Most of the complaints were from people who had eaten at the restaurant on January 13 or January 14 and became ill 12 to 24 hours after dining there, said the health department in a posting on its website. Health officials are calling about 100 Harbor Inn patrons in a case-control study to find out what they ate and how they may have become sick.

"We are not sure yet how it started," Harbor Inn Seafood Manager George Ciogas told CNN in a telephone interview.

Following the initial report of illness, Catawba County Public Health's Environmental Health specialists said they conducted a thorough inspection of the restaurant and found no violations. "There does not appear to be any concern about ongoing exposure to norovirus at Harbor Inn," said Doug Urland, health director of Catawba County Public Health Department, in a statement. "A case-control study will give us and state health officials the ability to learn more about the nature of norovirus, how it spreads, and what steps may be available to help better prevent this type of illness in the future."

But health officials acknowledge they may never be able to track down the cause.

"The most important message we have right now is that people who are ill with vomiting or diarrhea should not work, go to school or attend day care while they are having symptoms," said Dr. Megan Davies, the state epidemiologist. "Everyone needs to wash their hands frequently and thoroughly with soap and water. This is the most effective way to protect yourself and others against norovirus, since hand sanitizers alone are not as effective against this hardy virus."

Common symptoms of norovirus infection include nausea, vomiting, diarrhea and stomach cramps, according to the Centers for Disease Control and Prevention. Some victims may have fever, chills, headache, muscle aches and a general sense of tiredness, it says on its website. The CDC estimates that more than 20 million cases of acute gastroenteritis are caused by noroviruses in the United States each year.