Babies at a second day care center have contracted measles following an outbreak linked to Disneyland in late December.

Five children too young to be vaccinated in a Chicago-area day care center have contracted the contagious virus, Illinois health officials said Thursday. The announcement comes after a baby at a Santa Monica, California nursery also became infected, prompting the center to temporarily close and 14 babies to be voluntarily quarantined.

The infected children are all under one.

Illinois and Cook County departments of public health released a statement confirming the cluster of infected babies at the KinderCare Learning Center in Palatine, a Chicago suburb. It follows the confirmed case at Samohi Infant Toddler Center in Santa Monica on Feb. 2.

“Laboratory testing has confirmed a measles diagnosis for two of the children. Test results for the three remaining cases are still pending, but have been diagnosed based on clinical and epidemiological criteria,” the Illinois and Cook Country health departments said.

The source of the cluster is not yet known, and health officials called on any resident who is unvaccinated and experiences symptoms of a high fever and a rash to seek medical assistance.

The parents of the infected babies from Palatine had not opted out of receiving vaccination, but rather the children are too young for the first dose of MMR, the vaccine for measles, mumps, rubella. Babies under the age of one should be protected by the rest of the community who are immunized— what experts refer to as “herd immunity”—as long as 90% of the population are inoculated.

The diagnosis of the children follows an outbreak that started in December, with at least 40 people who visited Disneyland in Orange County, California contracting measles. There have been more than 100 confirmed cases as the highly-contagious disease spread across more than a dozen states, with the majority of patients in California.

The Centers for Disease Control and Protection (CDC) said the majority of cases were due to the infected person not being immunized for various reasons including medical, philosophical or religious. The rates of non-vaccination are increasing due to an anti-vaccination movement, which runs on the principle that immunization does a person more harm than good, despite no scientific evidence. Experts say the current outbreak is the inevitable consequence of the movement, although California health officials have said that the initial infected person was from outside the country.

“When you have only an extremely small proportion of the population vulnerable, herd immunity will protect them,” Anthony Fauci, director of the National Institute of Allergy and Infections Disease told Fortune. “When you have a substantial proportion, like the 10%-plus that we’re seeing in certain communities in California, then herd immunity doesn’t work very well.”

Measles is a respiratory disease that although not often deadly causes shocking symptoms. An infected person may suffer from a characteristic rash normally along the hairline, before becoming ill with a fever, red and sore eyes, a runny nose and a cough. The disease can result in pneumonia and is transmitted by coughing and sneezing.

Health officials have called on those who are not immunized to avoid places where large numbers of people gather and to consider immunization immediately.

“Measles is highly contagious and highly preventable through vaccinations,” according to the California Department of Public Health. “CDPH is urging caution to individuals who are not vaccinated, especially infants under 12 months. Any place where large numbers of people congregate and there are a number of international visitors, like airports, shopping malls and tourist attractions, you may be more likely to find measles, which should be considered if you are not vaccinated.”

Children must be at least a year for the first dose of the MMR vaccine, with the second dose administered normally before the child starts school. Two doses are 99% effective at preventing infection, according to the California Department of Public Health, although the CDC reports the full vaccination is 97% effective. Every state in the U.S. requires vaccination against measles, yet all have exemptions based on a variety of medical, religious or philosophical reasons.

In California, which currently has 99 confirmed cases of measles, 3.3% of kindergarten students, or 18,270 children, had not been vaccinated at the start of kindergarten in the 2013-2014 school year. Of these children, 3.1% or 17,253, were not vaccinated for “philosophical reasons,” according to figures from the CDC. In light of the current situation, a law has been proposed that would put an end to avoiding vaccinations on non-medical grounds in the state.

On Wednesday, two California state senators — Democrats Ben Allen and Richard Pan — said they plan to introduce legislation that would force parents to vaccinate their children due to personal beliefs, Reuters reported. The anti-vaccination movement has been spurred by debunked science claiming the MMR vaccine is linked to autism.

“The high number of unvaccinated students is jeopardizing public health not only in schools but in the broader community,” Allen said in a statement. “We need to take steps to keep our schools safe and our students healthy.”

Although the disease is still common in other parts of the world such as Europe, Asia and Africa, 2014 saw the greatest numbers of cases of measles since elimination in the U.S. in 2000 and more cases are expected.

“There will be more cases. We shouldn’t be surprised about that,” Dr. Terry Mason, chief operating officer of the Cook County Department of Public Health, said on Thursday. “The cat is out of the bag.”

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