Health officials in Illinois' collar counties are keeping a close eye on the number of whooping cough cases recorded in their communities.
McHenry, Lake and DuPage counties have seen an increase in the number of cases so far this year, with the illness affecting nearly 500 residents, the Chicago Tribune reported.
McHenry County, for example, has seen as many as 160 cases of whooping cough, including cases in more than two dozen schools. New cases there number 30 to 40 each week.
The state of Illinois reported just over 1,000 cases as of last week, compared to 1,057 cases in all of last year.
Health officials say many people don't even realize they have whooping cough, which is highly contagious, because its symptoms mirror those of a common cold.