All seven measles patients are unvaccinated, according to the Cook County Department of Public Health.
The cluster of cases occurred at a KinderCare in Palatine, Illinois, prompting the daycare chain with 1,900 locations nationwide to change its policy on Friday, requiring all employees working with infants to be vaccinated against the measles.
The first dose of the MMR vaccine, which protects against measles, mumps and rubella, is not administered until a child is a year old, and KinderCare said in a statement that it hopes to protect its children who are too young to be vaccinated by also limiting access to infant rooms.
Health officials in Illinois were still trying to find how the virus got to the daycare over the weekend.
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.