( CNN ) -- 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 .
At least 135 people sickened in one outbreak linked to a restaurant . The CDC says the number of cases is expected to rise as more illnesses are reported and new ones emerge. `` We are not sure yet how it started , '' Harbor Inn Seafood manager says .