Ten of the city's cases--which include both adults and children--are linked to an outbreak at a migrant shelter.
Local health workers are offering the MMR vaccine at other shelters as well as at the city's landing zone.
Chicago health workers have vaccinated hundreds of shelter residents, as Arizona health officials investigate cases near Flagstaff.
Illinois and California are among 17 US jurisdictions that have reported measles cases this year.
The CDC said today that it has received reports of 45 cases from 17 jurisdictions so far this year.
Michigan has reported 2 more cases from 2 different counties, part of a small but steady rise in US cases.
Also, Florida and Ohio have reported more cases.
So far, 15 US jurisdictions have reported 35 cases this year, including 15 in just the last week.
In other US measles developments, Arizona's Maricopa County reported two more cases.
The infections are occurring amid a global rise in cases and gaps in MMR vaccination.