Illinois health officials are still trying to figure out how five infants at an Illinois daycare center became infected with the measles virus.
Interested in ?Add as an interest to stay up to date on the latest news, video, and analysis from ABC News.
The infected infants are all too young to be vaccinated against the contagious virus.
While the first infant was reported to be sick last Sunday, the source for the Illinois measles outbreak remains undetermined, according to a spokeswoman for the Cook County Health Department. While a multistate measles outbreak that originated at Disneyland has led to at least 102 infections in 14 states, officials could not confirm that the outbreak was related to the cases in Illinois.
An adult in Cook County was also diagnosed with measles earlier this month, but the department has not found a clear link between that case and the cluster of infants who were sickened.
To stop the outbreak, Cook County Health officials have asked the families of other infants who were exposed to the virus and not yet vaccinated to stay at home.
On Friday, the KinderCare company, which ran the daycare center in Palatine, Illinois, announced that all employees at their 1,900 centers would have to be vaccinated against measles if they worked in infant rooms.
Measles is one of the most contagious viruses in existence and can infect 9 out of 10 exposed people if they are not immunized. The virus can cause rash, fever, conjunctivitis and runny nose. In severe cases the virus has caused swelling of the brain, pneumonia or even death.