Aegis Living of San Rafael reported 25 cases of the gastrointestinal illness, 22 of them affecting residents, said Shanna Cronan, senior public health nurse of the Marin County Department of Public Health's Communicable Disease and Prevention and Control program.
The first case at the San Rafael location occurred Jan. 14 and the facility at 111 Merrydale Road reopened today after no new cases were reported, Cronan said.
Aegis of Corte Madera, which reported 62 cases starting Jan. 18, remains closed and is discouraging visitors, Cronan said. Thirty-nine of the residents at the facility at 5555 Paradise Drive contracted the illness that includes vomiting, diarrhea and nausea, she said. The last case was reported Tuesday, she said.
Both facilities were closed except for emergency visits, Cronan said.
"They typically close until there are no new cases for 96 hours," Cronan said.
A spokeswoman for Aegis Living of San Rafael did not respond to a call for comment today.
Seventeen cases of the norovirus also were reported at a preschool in Marin County beginning Jan. 13, Cronan said. It affected 16 children and an adult and the school is closed today, Cronan said. Cronan would not identify the school.
The gastrointestinal illness occurs year round but is more common in winter when people are indoors more, Cronan said.
"It's the most common gastrointestinal illness in the United States. We see more and more of it every year," she said.
The virus was identified in 1984, Cronan said. It spreads from person to person by contacting objects such as doorknobs, computer keyboards and then putting your hands to your mouth, Cronan said.
The illness can also spread through the air when someone vomits, she said. The particles of vomit in an aerosol form can enter other people's mouths, she said.
"It's a hardy organism," she said.
Norovirus outbreaks must be reported within 24 hours, Cronan said. The Aegis facilities delayed reporting the outbreak until Jan. 20, she said.
Rebecca Ng, deputy director of Marin County's Division of Environmental Health, said she learned about the outbreaks at the facilities Monday. She said she was sent an email about them on Friday but she neglected to read it before she left work.
She said her agency is not investigating the outbreak.