With many of California's swine flu cases and investigations among children, officials are giving special attention to the rich breeding grounds that are schools.
Education officials are urging parents of the state's 6.3 million students to keep sick kids at home but keep sending healthy children to classes.
Arthur Reingold, the head of the Division of Epidemiology at the University of California, Berkeley, said children are a common conduit for disease so schools are a good place to focus.
"In influenza viruses of any kind, much of the transmission in the community happens among school-age children," he said.
The problem lies in students, especially very young ones, who lack hygiene skills such as keeping coughs and sneezes to themselves, said Dr. Bonnie Sorensen, chief deputy director of the California Department of Public Health.
"Our schools are the germinators," she said.
State health officials said that as of Tuesday night there were 11 confirmed swine flu cases in California. Individual counties have reported other cases that have yet to be confirmed by the state.
So far just one public school and a few private schools have closed in California because of confirmed swine flu cases or investigations into illnesses.
Teachers at Highlands Elementary School in Pittsburg contacted parents Tuesday night to tell them the school will be closed for a week after tests from Contra Costa County health officials revealed three probable cases among fourth-graders, superintendent Barbara Wilson said. The cases had yet to be confirmed by the state.
County health departments are tracking cases and advising schools whether to close, Sorensen said. The state also is making hygiene instruction materials available for schools to post in classrooms.
School authorities, meanwhile, say parents shouldn't keep healthy children home.
"Our schools are safe. We want students to continue to come to school," said Jack O'Connell, state superintendent of public instruction.
San Diego and Imperial counties have five confirmed cases each of swine flu.
In Imperial County, a 4,500-square-mile swath of southeastern California neighboring Mexico, schools Superintendent John Anderson said families of students regularly cross the border. Imperial County residents fear they are at risk because of the proximity to the country where there have been about 150 suspected swine flu deaths.
"I've heard some people talk about this like it's Hurricane Katrina: There's something out there, we think it's going to be big, it has potential to be bad," said Anderson. "Not that you're ever fully prepared, but we want to be prepared."
A private Lutheran elementary school in San Diego, Christ the Cornerstone Academy, was closed Tuesday after a 7-year-old boy was confirmed as having swine flu, said Jose A. Alvarez, a spokesman for the San Diego County Health & Human Services Agency.
It was not immediately clear how the boy contracted the illness. The boy's father and sibling also are suspected of having the flu, said Alvarez.
State officials confirmed one swine flu case in Northern California. Officials in Sacramento and Marin counties each reported two additional cases, and Santa Clara and Tulare counties each reported one.
Sacramento County health officials said a teenage student at St. Mel School in Fair Oaks who traveled to Mexico did not test positive for swine flu, but two other students did. The school will be closed for the rest of the week.
In Marin County, officials said a 60-year-old woman and her 20-month-old granddaughter tested positive for swine flu after a Mexico vacation that took them to Cancun, Cozumel and Playa del Carmen. They were not hospitalized but were staying home.
