Wisconsin officials are scratching their heads trying to figure out how the state led the nation in the number of whooping cough cases last year.
Wisconsin had 4,048 whooping cough cases last year, nearly double the next closest state, New York, with just over 2,000.
For many states, the number of cases was in the single figures.
Dan Hopfensperger is the director of the Wisconsin Immunization Program.
He says the state is examining the way it identifies and investigates cases and is comparing notes with other states.
He suspects that other states aren't as aggressive as Wisconsin in keeping track.
Doctor Matt Hall, a Marshfield Clinic infectious disease specialist, says the state's high number of cases could be due to new testing and more testing.
The state introduced a new molecular test in the last few years that he says probably doubles the number of cases you can find.