Montana health workers report more than 200 cases of whooping cough so far this year in Montana - the most since 2005, when the outbreak totaled 600 cases.
Since January, pertussis has spread to 18 counties and tribal health jurisdictions.
Gallatin County, Lewis and Clark County, Flathead County and Ravalli County are among the hardest hit.
A total of 52 cases have been confirmed in Ravalli County, with 12 reported in the last two weeks.
Flathead County reports 38 cases, with 34 reported in the last two weeks.
Missoula County has eight cases.
Several states are also reporting increases. Washington state is leading the way with 13,000 cases reported so far this year.