Officials believe about 10 other children at the center may have been exposed.
In a joint statement, the Illinois Department of Public Health and the Cook County Department of Public Health said the cluster includes five infants at KinderCare Learning Center in Palatine.
All five children are under the age of one -- too young to have gotten vaccinated.
Lab tests have confirmed measles cases in two of the children; the other three have been diagnosed with measles based on their symptoms while doctors await lab tests to confirm the diagnoses.
"Individuals who are under the age of one or with certain clinical conditions cannot be vaccinated and are therefore at highest risk for measles. Residents are strongly encouraged to get vaccinated to protect themselves and the most vulnerable members of the community," officials said.
Students, staff, and faculty at the day care center have been notified. Anyone who hasn't received a measles vaccination has been told to stay at home and away from unvaccinated people for the next 21 days.
"While these measles cases seem to be focused on the northwest suburban Cook County region, any resident who is unvaccinated and experiences symptoms of a high fever and a rash should call their local health department as well as their healthcare provider," the statement said. "These individuals should notify their doctor or emergency department before seeking care so that staff are able to take appropriate precautions to prevent others from being infected."
Officials said the source of the infection is unknown, and there does not appear to be a connection to the measles outbreak linked to Disneyland.
Symptoms of measles include fever, red and sore eyes, runny nose, cough, and rash. Measles can cause more severe health problems, including pneumonia, encephalitis, and even death. The virus is transmitted by coughing and sneezing, and can survive in the air and on surfaces for up to two hours. People who have measles are contagious from four days before a rash starts, through four days afterward.
"This situation continues to underscore the importance of getting vaccinated. Vaccinations are the safest, most effective way to protect individuals from measles and other potentially dangerous communicable diseases," officials said.