JACKSON, Wyo. (AP) — A norovirus outbreak believed to have started at Yellowstone National Park may have affected around 200 people at both Yellowstone and Grand Teton National Park.

About 50 visitors have reported symptoms associated with norovirus, a stomach flu that is easily spread by touching an infected person or contaminated surfaces. Up to 150 park employees may have been infected, though not all those cases have been confirmed. The outbreak is believed to have started with a group of tourists who visited the Mammoth Hot Springs area in Yellowstone on June 7.

They complained of stomach flu symptoms and, within 48 hours, employees who work with visitors also reported being sick. Physician's assistant Michael Takagi told the Jackson Hole Daily that the outbreak is one of the most significant ones he's seen.