Five infants under the age of 1 have contracted the measles virus at a suburban Chicago, Ill., daycare facility. The five infants add to the more than 100 people already infected with the disease throughout the country, which started in mid-December and has spread to 15 states.

Children under 12 months of age are particularly at risk for contracting the virus, since the first dose of the measles vaccine is typically not given until a child reaches the age of 1. None of the infected children have parents who have opted out of the vaccine, but are simply too young to have been inoculated yet.

The Palatine KinderCare Learning Center was informed of the first two infections on Thursday, after laboratory test confirmed the results. Results on the other three infants who are ill are still pending. According to Cook County Department of Public Health’s Dr. Terry Mason, more cases should be expected.

Children and staff at the daycare have been notified of the outbreak. Anyone who has not been vaccinated with the MMR shot (measles, mumps and rubella) has been told to stay at home. Colleen Moran, KinderCare spokeswoman, says that anyone at the facility without an updated vaccination record is “excluded from the center” until Feb. 24.

Young children are the most vulnerable to measles, according to Dr. Wendell Wheeler of Ingalls Memorial Hospital. He says the thick mucus associated with the disease and the small airways in the very young “don’t go together.” Pneumonia is often a complication.

There has been one adult in Illinois has been diagnosed with measles. The suburban Cook County resident has been identified by officials only as being older than 18. Three locations have been listed where others may have been exposed to this patient, and two of them are in Palantine. Officials said that it is not known whether the adult’s measles diagnosis is linked to either the Disneyland outbreak or the cases at the daycare center.

The Illinois health department has not yet identified the source of the measles infection at the daycare center. It is an extremely contagious disease that infects 90 percent of unvaccinated people who are exposed to the virus. In addition to a red rash, symptoms include runny nose, fever and conjunctivitis.