State health officials are urging parents to make sure their children have received all their recommended vaccinations.
The Illinois Department of Public Health says the state is reporting its highest number of pertussis cases since 1950.
In 2012 there were 2,026 cases of the illness, also known as whooping cough.
Dr. LaMar Hasbrouck, director of the health department, says the record number of cases is "the perfect example of the importance of continued immunization."
Saturday marks the start of National Infant Immunization Week.
Hasbrouck says parents may visit the department's website to find a list of recommended vaccines and the age at which children should receive them. Illinois residents who cannot afford to pay for the immunizations may be eligible for assistance.