Santa Clara County public health department leaders reported a third case of coronavirus (COVID-19) within the county on Friday, the same day Palo Alto school district officials learned a parent of two students may have been exposed to the virus. The two students, who attend Palo Alto High School and JLS Middle School, were sent home as a precaution, Superintendent Don Austin said.

The third coronavirus case involves an older woman who has chronic health conditions and was hospitalized for a respiratory illness, according to a department press release.

The patient received treatment at El Camino Hospital in Mountain View, according to Richard Angeloni, interim communications director at the hospital. It doesn't appear she was exposed to the virus through travel or had contact with a traveler or someone with the virus.

The hospital is working closely with the Santa Cara County Public Health Department, California Department of Public Health, and Centers for Disease Control and Prevention, he said in a media statement. The hospital could not provide additional information about the patient due to federal healthcare privacy laws.

"The health and well-being of our patients, visitors, and employees are our main priorities, and we take our responsibility to provide a safe environment and the best care very seriously," he said.

At a press conference on Friday in San Jose, Dr. Sara Cody, the county's health officer, said the woman's doctor had contacted the public health department on Wednesday night to report the suspected case after the woman had trouble breathing. The department received the specimen on Thursday, and results later that night showed the woman tested positive. There is no evidence that the woman's case is linked to other cases in California, Cody added.

"The department has been working to identify contacts and understand the extent of exposures," Cody said.

"This new case indicates that there is evidence of community transmission but the extent is still not clear," she said in a statement. "I understand this may be concerning to hear, but this is what we have been preparing for. Now we need to start taking additional actions to slow down the spread of the disease."

"This investigation is just beginning. This case does signal a shift in tactics," she added.

Isolation and quarantine, which the county has implemented for at least the past five weeks, have helped to slow the spread of the virus, but "now we need to add other health tools to the mix," she said.

The county's public health lab now has testing kits from the U.S. Centers for Disease Control and Prevention and assistance teams have arrived at the county's emergency operations center from the California Department of Public Health and the CDC, she said.

Austin said he did not know whether the parent was the same person announced by the county but that to his knowledge, the Palo Alto parent had only been exposed and not infected by the virus. He said travel to China was not a factor in this case.

Lana Conaway, the district's assistant superintendent of equity and student affairs, said that the district learned from another parent that this parent "was reported to have been in public proximity to an infected person" but that "there is no indication of infection at this time." Austin said the district confirmed with the parent involved that he or she had been exposed.

Austin said he has not been notified of any mandatory quarantines of school district students or parents. (Self-quarantines are harder to monitor because there is no requirement they be reported.)

"We have 11,000-plus students with about 20,000 parents, another 2,000 employees. That's 35,000 individuals that each have some giant number of contacts. The school district, when you really play it out, literally has potentially millions of contacts. It's unreasonable to think we're going to be any better prepared to limit or control those contacts," Austin said Friday.

"What we can do," he added, "is make sure that we're on top of any reported symptoms or verified cases and handle those appropriately. Right now, at this time, we have zero."

Conaway said the district is planning "aggressive" cleaning of all hard surfaces, both inside and outside, at Paly and JLS.

"Although we already had protocols in place to address the coronavirus we are amping up our vigilance ... particularly at those two sites that have been identified," she said.

She encouraged parents and students to wash their hands often and to stay home if they have any symptoms, including fever or respiratory distress.

The district has formed a team to assess the situation and provide information as possible, Austin wrote in his message to parents on Friday. Officials also are preparing a frequently asked questions page. The district is continuing to follow guidance from the California Department of Public Health and the CDC.

"We recognize that the unknown can be concerning and will continue to provide regular updates as we have them," Austin wrote to parents.

Hours earlier, the district had sent out a message about preparing for the potential risk of the spread of coronavirus in Palo Alto.

The virus has sickened more than 80,000 people worldwide, mostly in Wuhan, China. The news comes nearly a month after the county's first two cases of coronavirus were made public.

The first case was reported on Jan. 31 and involved a man who traveled from China and has since recovered. In the second case, which was reported on Feb. 2, was a woman who came to the county from Wuhan on Jan. 23 to visit family. She has stayed at the home since her arrival, with the exception of two occasions when she sought outpatient medical care, public health staff said.

In the first two cases, the patients had mild symptoms and did not require hospitalization but they remained in quarantine, according to public health leaders. The three cases don't appear to be related to each other.

In light of the coronavirus outbreak, public health leaders have provided several recommendations the public can use as a precaution. People can wash their hands to avoid getting sick or spreading germs; covering their cough; staying home if they're sick; avoiding people who are ill; and begin making plans if a family member becomes infected.

School officials can consider ways to deliver lessons through "tele-learning" and make extra efforts to clean surfaces, according to the Public Health Department. Businesses can consider substituting in-person meetings for video or telephone conferences, providing teleworking options, altering their absence policies and increase surface cleaning.

Although California health leaders were initially frustrated by a dearth of testing kits, eight public health labs can now test for coronavirus, including the state microbial diseases lab in Richmond and county labs in Alameda, Santa Clara, Tulare, Ventura, Los Angeles, Orange and San Diego counties, said Dr. Christopher Braden, deputy director of the National Center for Emerging and Zoonotic Infectious Diseases at the CDC. More testing kits are being shipped this week to other labs, he said.

When someone tests positive, they will be tested serially until their results return negative, he said. The virus can survive on surfaces for days, but it is highly susceptible to cleaning products, he said at the press conference. He noted, however, that the likelihood of contracting the virus from a surface where it has been for days is small. The known transmission is person to person through droplets and aerosols from coughs and sneezes that enter through eyes, noses and mouths.

The county Public Health Department held a press conference to discuss the third case at 4 p.m., which was livestreamed from its Facebook page.

The city of Palo Alto has posted a webpage with information and local updates on the coronavirus at cityofpaloalto.org.

East Palo Alto officials in a press release on Friday said they are closely monitoring the coronavirus developments and are coordinating with San Mateo County health officials and the CDC.

Residents in East Palo Alto can learn about the virus and find updates in their city and county on the San Mateo County Health website at smchealth.org/coronavirus.

This story will be updated as more information becomes available.

Jamey Padojino and Kate Bradshaw contributed to this report.

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