Tomales Bay oysters infected with a norovirus prompted the California Department of Public Health to intervene early this month. On Jan. 4, the public health department shut down oyster harvesting and recalled a batch of oysters sold by Hog Island Oyster Company to a host of West Marin and Bay Area restaurants. The closure remains in effect, though the farm is open and selling product from other regions. Dr. Matt Willis, Marin County’s public health officer, said the virus was likely in the water itself. He confirmed 44 cases of norovirus primarily between Dec. 29 and Jan. 5 across the Bay Area; seven were Marin residents. Based on interviews with the sick people and stool samples, the health department pinpointed the virus and its source. Dr. Willis said it is uncommon for a norovirus outbreak to be linked to oysters; the last instance was 20 years ago. State health officials are now continuously sampling oysters in the bay for norovirus, as well as, for vibrio parahaemolyticus, the bacteria that more often causes oyster-related foodborne illness. The state will determine when to re-open the bay, which it typically closes intermittently during the rainy season.