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Travelers are seen wearing protective masks while arriving from an international flight into Houston International Airport on Tuesday, April 28, 2009 in Houston. (AP Photo/Houston Chronicle, Julio Cortez)

Contra Costa County health officials decided to close a Pittsburg elementary school for a week beginning today, after tests there revealed three probable cases of swine flu.

Teachers at Highlands Elementary School were contacting their students' families Tuesday night to inform them of the decision, said Barbara Wilson, superintendent of the Pittsburg Unified School District.

Thirteen children from the school were either sent home or called in sick Tuesday with flu-like symptoms.

The fourth-graders, including one with a relative who recently visited from Mexico, all are in the same class. The school notified county officials.

The county then quickly dispatched public health nurses to the children's homes to take nasal swabs. The county laboratory analyzed the specimens and concluded that at least three are probable swine flu cases.

"So far, we've only had reports of very mild illnesses," said William Walker, director of Contra Costa Health Services. But he said the school is being shut down until May 6 as a precaution.

Custodians spent the afternoon sanitizing the classroom and other areas the students may have frequented.

Earlier in the day, Gov. Arnold Schwarzenegger declared a state of emergency. A short time later, health officials in Marin and Santa Clara counties announced what were believed to be the Bay Area's first swine flu cases, along with the ones in Contra Costa County.

As the virus continued to spread, state and federal health authorities labored to discover the outbreak's scope and to determine the best response for protecting public health.

No confirmed deaths have been linked to the virus in the United States. But more than 150 people have died in Mexico and officials here are uncertain how severe the outbreak will be.

"With a new infectious agent, you don't sit back and wait and hope for the best," said Richard Besser, acting director of the national Centers for Disease Control and Prevention. "You take bold steps and then you pull back if you need to. We are in a pre-pandemic period."

In Marin County, a 60-year-old woman and her 20-month-old granddaughter, who recently traveled to Mexico, became moderately sick after returning home. Both have probable swine flu cases.

The child has since recovered. The woman now has only mild symptoms, but as a precaution, the county tested other household members and gave them medication to help ward off illness.

Santa Clara County also announced a probable case. The 16-year-old female had recently traveled to Southern California. Contra Costa County also has tested an ill student from Riverside High School in Pittsburg, but those results were not available Tuesday night.

Health officials throughout the East Bay have been holding daily conference calls to track the disease and determine the best response.

"We're waiting to see the progress of what so far has been a mild illness with hardly any hospitalizations, let alone fatalities," Walker said.

Experts urged people not to panic, but to take precautions to protect themselves, including frequent hand-washing, covering a cough, and staying home from work or school if feeling ill.

Alameda County this week submitted 39 specimens for testing and should have the results within 24 hours, said health department spokeswoman Sherri Willis.

Numbers changed constantly, but as of Tuesday afternoon, state officials said they had 11 confirmed swine flu cases — five in San Diego County, five in Imperial County and one in Sacramento County. They counted seven probable cases: two in the San Diego area, two in Marin, and three in Sacramento County.

All of the Sacramento cases have been linked to St. Mel School, which was temporarily closed, officials said.

Nationwide, the CDC reported 66 confirmed cases in New York, Ohio, Kansas, Texas and California. Five people have been hospitalized, including three in California, but the California people have all been released and are recovering.

In his emergency declaration, Schwarzenegger ordered all agencies to coordinate with public health officials as needed, said competitive bidding on contracts could be suspended to deal with the outbreak, and waived certification requirements for laboratories involved in the testing.

"There is no need for alarm," the governor said. "We are taking strong and swift action to limit the spread of the virus and minimize its effects."

The state has spent several years preparing for an outbreak and has resources available if it should worsen, said Bonnie Sorensen, chief deputy director of the California Department of Public Health.

California has its own stockpile of antiviral medications and will be getting more from the CDC. It also has 21,000 hospital beds that can be deployed to communities as needed, 2,400 ventilators, and three mobile hospitals with 200 beds each.

The state also has obtained the equipment and chemical substances to do testing at its Richmond laboratory beginning this week. That should provide faster results because the state will no longer have to send specimens to the CDC in Atlanta.

With all the publicity about the outbreak, many people are contacting their doctors at the first sign of illness. Kaiser Permanente's switchboards are "experiencing substantially higher call volumes than expected for this time of year," said Stephen Parodi, chief of infectious disease.

More patients than normal also have been showing up this week with colds, sore throats and coughs at Alta Bates Summit Medical Center in Berkeley and St. Rose Hospital in Hayward, officials said.

Staff writers Matthias Gafni, Doug Oakley, Chris Metinko, Suzanne Bohan, Tom Lochner, Paul Burgarino and Eric Kurhi contributed to this story. Reach Sandy Kleffman at 925-943-8249 or skleffman@bayareanewsgroup.com.