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The impact of coronavirus on the Philadelphia region intensified Monday as new cases emerged, tens of thousands of students and staff were told to stay home from dozens of area schools, and universities, businesses, and trade groups canceled or postponed events and classes.

Three new cases were announced in Montgomery and Monroe Counties, bringing Pennsylvania’s total to 10. Three of those patients are hospitalized. New Jersey identified five new patients, most in North Jersey, bringing its total to 11, and Gov. Phil Murphy declared a state of emergency and a public health emergency.

Seven of Pennsylvania’s diagnosed patients are in Montgomery County, where a growing list of schools announced they would be closed for deep cleaning, had canceled field trips, and began making preparations for remote teaching. Along with closures in Chester, Bucks, and Camden Counties, the affected districts serve more than 30,000 students.

“We expect more cases to be confirmed in the upcoming days and weeks,” said the Pennsylvania’s secretary of health, Rachel Levine, “but we want everyone to take action to prevent the spread of this virus.”

Because of the virus’ 14-day incubation period, during which an infected person could be symptom-free but still contagious, the domino effect from just a single diagnosis is significant. After a cardiologist at the King of Prussia campus of Children’s Hospital of Philadelphia tested positive on Monday, at least six area school districts announced closures.

Nationwide, the number of cases grew, and the stock market continued its plunge. On the West Coast, a cruise ship carrying 3,500 passengers and at least 21 infected people arrived in Oakland, Calif., where coronavirus patients were to disembark, and a second person died of the virus in California. On the East Coast, the executive director of the Port Authority of New York and New Jersey came down with the virus, as did a church rector in Washington who distributed communion on Sunday, leading that city’s mayor to ask hundreds of churchgoers to self-quarantine.

There has yet to be a coronavirus case in Philadelphia. One of the Montgomery County patients became the first to be treated in the city, in an isolation room at the Hospital of the University of Pennsylvania. The city was feeling the impact in other ways: At least two major conferences in Philadelphia expected to draw a total of 21,000 people were canceled, costing the city visitors and the region estimated millions in economic impact.

The American Chemical Society, the world’s largest scientific society, for the first time in more than 100 years canceled its annual meeting, which was expected to draw 15,000 visitors the week of March 22. The American Academy of Allergy, Asthma & Immunology canceled its annual meeting of about 6,500 people. Both were to take place at the Pennsylvania Convention Center. The University of Pennsylvania also canceled its annual Penn Wharton China Summit, whose average attendance is 1,500, and other smaller professional gatherings around the region were also nixed or postponed.

Pennsylvania officials were not recommending that public events be canceled, but Levine urged anyone who is sick not to expose others.

“Please stay home if you are sick,” she said. “Please stay home from social events. Please stay home from work. Please stay home from school.”

Many were heeding the advice. Cheltenham School District, with about 4,300 students, closed its seven schools for the rest of the week after learning a parent in the district had been a caregiver to someone with a presumptive case of the virus.

“I do not want to cause panic; however, I must prioritize the interests of our students and staff,” wrote Cheltenham Superintendent Wagner Marseille in an email to families and staff, saying the parent “has self-quarantined herself and her child.”

It joins Germantown Academy, which on Sunday announced it would close to its 1,200 students for most of March because a family member of a student at the Fort Washington school tested positive.

Two school districts and three other schools said they would be closed Tuesday, all because students, parents or staff of the schools may have been exposed to the sick doctor during visits to CHOP. Norristown Area School District, with about 7,500 students; Lower Merion School District, with more than 8,600; Upper Merion School District, with nearly 4,200 students, all announced plans to close as well as four individual schools — Simmons Elementary in the Hatboro-Horsham; Neshaminy High School and Buckingham Friends School, both in Bucks County; and Henderson High School in Chester County.

In New Jersey, Haddonfield School District, which serves 2,700 students, canceled classes for March 16 so that staff could attend a daylong planning session on online classes. A number of South Jersey districts, including Cherry Hill, are also preparing plans to teach remotely; others conducted deep cleanings over the weekend.

Meanwhile, five schools in the Central Bucks School District reopened Monday after closures Friday for cleaning and after students and staff who had been exposed to coronavirus tested negative.

Universities began announcing extended spring breaks and preparations to move classes online. Rowan University extended its break and directed faculty to begin planning for online classes. Princeton said all lectures will be online from March 23 to April 5, urging students to stay home after spring break ends.

All made note of one key element: the uncertainty. This, as one university reminded its faculty, could last for “an indefinite period of time.”

The Montgomery County patients whose locations have been identified reside in Upper Merion, Lower Merion, Lower Gwenydd, and Worcester Townships.

As health officials trace the contact with others each sick person had in the last 14 days, they are notifying anyone who might have been exposed.

CHOP was contacting patient families who may have come into contact with the ill cardiologist over the last week and advising them to self-quarantine. Staff who came into contact with the doctor have also been instructed to self-quarantine at home for two weeks, CHOP CEO Madeline Bell said in a Monday email to staff obtained by The Inquirer.

Cases tested by the state are presumed positive until they have been verified by the U.S. Centers for Disease Control and Prevention. Levine said she did not know when the state would get the results from the CDC, which has not yet confirmed any of the cases in Pennsylvania or New Jersey.

At least five more people tested presumptive positive for coronavirus in New Jersey, most in North Jersey and one in Cherry Hill, bringing the state total to 11. Another 24 people are currently under investigation statewide, officials said.

“If all remain calm and informed, and educated with each passing day, and just practice common sense," Lt. Gov. Sheila Oliver said at a news conference, “the risk of an individual contracting coronavirus remains low.”

Staff writers Sarah Gantz, Oona Goodin-Smith, Pranshu Verma, Tom Avril, Susan Snyder, and Melanie Burney contributed to this article.