Chicago has seen 56 measles cases this year, accounting for over half of the cases across the country.

The Centers for Disease Control and Prevention also reports more than half of cases are in children under the age of 5.

The Chicago Department of Public Health says they are fighting the spread through testing and vaccinations — particularly in the Pilsen migrant shelter where the majority of cases are emerging.

Dr. Brian Borah, medical director for vaccine-preventable diseases at the Chicago Department of Public Health, says their efforts have led 6,000 people receiving vaccinations at 75 different vaccine events since March.

“I think we were able to get ahead of the virus in that way,” he said. “And we really have targeted most of the shelters housing these new arrivals in Chicago and we think have put a good dent in this outbreak.”

CDPH has not specified how many cases are from the Pilsen migrant shelter but they have said it is not the source.

“Clearly we’ve had a lot of cases come out of that setting,” Borah said. “But I think it’s an open question about how measles came to be in that setting… Certainly the measles was in Chicago before this outbreak started. So we think that there are somehow links to one of those two cases or another case that we’re not sure about, but we’re still investigating those details.”

Children under 5 are considered high risk of becoming very sick 

Someone with measles typically starts showing symptoms similar to a cold such as a cough, runny nose or red eyes. A rash, which often starts at the head and moves down to the rest of the body, then signals a measles case.

Measles is one of the most contagious diseases and children under 5 are considered at high risk of becoming very sick from it, according to Borah. But he added vaccination is the key to immunity. If someone is unsure of their vaccination status, a blood test can be used.