CHICAGO (CBS) — A woman who works closely with “medically fragile” special needs students at a Chicago Public School has tested positive for the coronavirus. She had recently traveled on the Grand Princess cruise ship, officials said Friday.

She is a special education classroom assistant at Vaughn Occupational High School in Chicago, which serves children with special needs, who “are medically fragile,” CPS CEO, Dr, Janice Jackson said. Classes are canceled at the school next week and the building will be sterlized, officials said. The woman, who is in her 50s and now the sixth diagnosed patient in the Chicago area, departed the cruise ship on Feb. 21 in California and returned home a few days later. She then returned to work on Feb. 25. Her last day at work was Monday. CPS officials are working with staff and parents to notify them of the development and will provide all needed testing and health care, Chicago health commissioner Allison Arwady said.

The Grand Princess continued to cruise after the patient disembarked, and it is currently off the coast of San Francisco with more than 20 confirmed cases.  The ship’s crew first discovered evidence of the COVID-19 disease, caused by the coronavirus, on March 4.

A top CPS source told CBS 2’s Brad Edwards that the aide had “intimate, hands on contact” with numerous “special needs, medicially fragile students.”

The woman’s condition has been stabilized, officials said. Her test was confirmed late Friday afternoon.

“I want to assure our CPS students and families that our city is working relentlessly to protect the safety and security of our students, faculty, and staff,” Mayor Lightfoot said. “Despite this case, the risk to the general public still remains low.”

Vaughn, 4355 North Linder Avenue,  provides a specialized education for high school students with cognitive, developmental and multiple disabilities, according to the school’s website. The staff directory lists 35 teachers, administrators and aides. The school has 212 students.

“Vaughn is a unique school with many complex needs,” said Jackson. “This is currently an isolated case.”

In a statement the Chicago teachers union urged CPS and elected officials to ensure the teachers and families don’t suffer financial hardship.

“CPS has assured us that Vaughn faculty and staff will not face any financial hardship, and will be compensated during the quarantine. Parents and guardians affected by this outbreak may need to provide around-the-clock personal care for their children, so we urge Mayor Lori Lightfoot and Gov. JB Pritzker to ensure they can do so without employment risk or household hardship. No one should lose their job due to this crisis.”

Earlier, Friday, the fifth patient was treated in isolation at Rush University Medical Center and was released Friday afternoon to home quarantine. He’s a college student from Vanderbilt University, who told his student newspaper that he’s fielding calls “every 30 minutes” asking about “every person I’ve been in contact with.”

Health care workers are taking precautions to keep COVID-19 contained, such as having infected patients go into separate entrances and stay in isolated rooms. That, along with tracking down those people who have in contact with the sick, isn’t that simple, one expert said.

This is the same protocol used for the other four Illinois patients, who are now out of the hospital and recovering.

Infectious disease expert, Dr. Sarah Cobey, says tracking down the first patient in Illinois is no easy task. It’s likely there are people who had a mild case of the virus and didn’t know it. “In all likelihood there have been multiple introductions into the state.” she said.

That brings into question the effectiveness of travel screenings. The college student just infected had taken a flight to Chicago O’Hare airport from Florence, Italy.

“We know that viruses fly,” said Ezike. ‘They can come from wherever they are to new locations; they don’t have to stop at customs and be kept out.”

Research from the University of Chicago and UCLA argues that travel screening for COVID-19 is mostly ineffective. So many people travel, especially through hubs like O’Hare, and people can carry the virus for a long time before they even show symptoms. That means many travelers can pass screenings and still be contagious.

“The disease presents really differently for different people,” said Cobey. “Some people appear to be infected but show very few symptoms if any.”