During January-February 2015, Cook County Department of Public Health led an investigation of a measles outbreak predominantly affecting infants at a child care center who were too young for routine immunization with measles-mumps-rubella (MMR) vaccine. Methods. Measles cases and contacts were investigated by Illinois public health officials. Cases were isolated for 4 days after rash onset. Exposed healthcare workers and child care center staff were required to provide documentation of receipt of 2 doses of MMR vaccine or laboratory evidence of immunity to return to work. Susceptible contacts were actively monitored for 21 days after exposure and provided postexposure prophylaxis (PEP) if certain criteria were met. Results. Fifteen confirmed measles cases were identified; 12 (80%) occurred in infants who were attendees of a child care center. Clinical misdiagnosis of 1 case allowed for continued transmission within the center. Twelve (86%) of 14 exposed infants at the child care center were diagnosed with measles; no other attendees or staff were infected. Five cases visited outpatient pediatric clinics during their infectious period, exposing 33 infants. Six exposed child care center staff and 3 healthcare workers did not have documentation of immunity available and were excluded from work until this was obtained. No healthcare-associated transmission was identified. Ninety-one contacts were actively monitored and 20 received PEP. Conclusions. This outbreak underscores the vulnerability of infants to measles, the need for early consideration of measles in susceptible patients presenting with a febrile rash illness, and the importance of immunity among individuals working closely with infants. (author abstract)