CHICAGO (Reuters) - Five babies at a suburban Chicago daycare center have been diagnosed with measles, adding to a growing outbreak of the disease across the United States, Illinois health officials said on Thursday.

Officials are investigating the cluster of measles cases at KinderCare Learning Center in suburban Palatine, said a joint statement from the Illinois and Cook County health departments. All the children are under 1 year old and therefore would not have been subject to routine measles vaccination, which begins at 12 months.

Public health officials have reported that more than 100 people across the United States have been infected with measles, many of them traced to an outbreak that began at the Disneyland theme park in Anaheim in December.

One adult in Illinois was previously diagnosed with measles. The diagnoses for the adult and two of the babies have been confirmed through laboratory testing, health officials said.

Test results for the other three cases are still pending, but they have been diagnosed based on clinical and epidemiological criteria, officials said. The source of the infection for the children was not immediately known.

"This just underlines the importance of everyone being vaccinated," said Melaney Arnold, a spokeswoman for the Illinois Department of Public Health. Measles is a highly contagious respiratory disease that causes fever, red and sore eyes, runny nose, cough and a characteristic rash.

The disease sometimes causes severe complications including encephalitis and death.

(Reporting by Mary Wisniewski; Editing by Will Dunham)