"The individual is an international traveler who spent time in several countries that are experiencing measles outbreaks," the city told ABC 7 by email.
That unnamed person may have exposed Chicagoans to the highly contagious virus on Thursday and Friday before healthcare providers alerted officials.
"Early signs and symptoms of measles include fever, fatigue, runny nose and red/watery eyes. After a few days, the characteristic skin rash emerges. Most people don't seek medical care in the early stages of their illness because they believe they have other more mild illnesses," CDPH said.
Now, the infected person "is in a location where they cannot infect others and will remain so until they are no longer able to spread measles," said CDPH, adding that the department is working with CTA, the Department of Aviation, UIC, the Centers for Disease Control and Prevention, and others to contact all exposed individuals and see what measures need to be taken.
Health officials confirmed a case of measles in Chicago on Saturday.
The Chicago Department of Public Health says it's working closely with the highest-risk locations to contact exposed individuals.
CLICK HERE to read the full statement from CDPH
Exposures may have occurred on May 16 on the blue line from O'Hare International Airport to the University of Illinois Chicago campus, Millennium Park and retail stores on State Street between Monroe and Randolph Streets, and on South Canal Street.
Official say on Saturday morning, exposures may have occurred at the UIC Student Center East.
"It's not uncommon for Chicago, because we are an international hub, to have travelers come in from other places who are infected with diseases like measles," said Chicago Department of Health Commissioner Dr. Julie Morita.
Those who think they may have been exposed should check their vaccine records or contact their healthcare providers to determine if they need to be immunized.
"Measles is a serious yet preventable disease through a safe, effective and universally available vaccine," Morita said. "Chicagoans should make sure their children and family members are up to date on vaccines now. Vaccination is the best way to protect against measles."
There have been great studies that have shown that the measles vaccine is very safe and very effective, and so the best thing parents can do for their children is to make sure they're vaccinated," Morita said. "And adults who haven't been vaccinated should get vaccinated."
Officials report that Chicago has one of the highest rates of measles vaccination in the country.
Symptoms of measles include fever, cough, runny nose, red eyes, diarrhea and a rash consisting of tiny, red spots. Pregnant people, those with weakened immune systems, and those who have not been vaccinated are at highest risk from measles.
"Measles when it becomes really severe can lead to serious lung damage. It can lead to brain swelling. It can lead to blindness if it gets really bad," Morita said.
For more information about measles, contact your health care provider or visit www.cityofchicago.org/measles.
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