"We haven't had an outbreak this big in quite some time" said Matt Kelley, health officer with the Gallatin City-County Health Department.
Pertussis, known more commonly as whooping cough, is a bacterial infection that's highly contagious.
Bozeman High School has been hit hardest, where 13 students and a coach have come down with the illness.
"What we're seeing is kids in the high school that haven't had a booster in quite some time" Kelley said. "Their immunity may be waning and they might get sick."
Kelley said it could be hitting the high school hard because the vaccine and booster shots don't provide complete immunization.
But while it doesn't completely protect everyone, it could lessen the severity of the illness.
"If somebody identifies you as a close contact, we're going to call you" Kelley said, about the health department's actions to stop the outbreak.
They're contacting anyone who may have come in close contact with the 15 infected.
Whooping cough can spread quickly. The CDC says family members have a 90 percent chance of getting sick if they live in the same house as someone who's caught pertussis.
"The county health nurses and all our school nurses have done a great job explaining the seriousness" said Bozeman Public School Superintendent Kirk Miller. "If you have symptoms, you have to be straightforward with that, and then here is the antibiotic."
Miller said the school is doing the best it can to contain and stop the disease. Health workers put 350 high school students on anti-biotics.
But Kelley said it may take time to get the illness out of Gallatin County.
"It tends to be, when you get an outbreak this size, it's really tough to beat it out of the community" he said. "Some times it just has to run it's course."
Younger patients with whooping cough have a greater chance of developing complications than older patients.
The most common complication is secondary bacterial infection, which is the cause of most pertussis-related deaths.
Pneumonia occurs in one out of 20 cases.
Because of the restrictions, Bozeman Deaconess Hospital has put visitor restrictions in place.
The hospital is asking anyone with a cough or fever to not visit the hospital.
Patients of physicians in the Highland Park medical offices can continue to see their doctor and keep existing appointments.