The U.S. had more than 41,000 provisionally reported cases of pertussis in 2012, leading to 18 deaths, and the CDC is looking at what caused the increase in whooping cough. CDC officials said evidence does not show bacterial strain changes have created an increase in cases or made vaccines less effective but added a "waning immunity" may be behind the spike. To date, case totals for 2013 are well behind those for 2012.

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