JACKSON, Wyo. (AP) — Officials report an outbreak of norovirus in the Mammoth area of Yellowstone National Park this week.

Yellowstone spokesman Al Nash says the bug is thought to have been brought in by visitors on a couple of tour buses who were ill when they entered the park. Mammoth is on the northern end of Yellowstone near the Montana border.

Nash tells the Associated Press that the gastrointestinal illness peaked last weekend and is on the decline now. It infected about 30 people.

Before lab tests confirmed norovirus, Yellowstone staff and personnel decided early on to treat the situation as if it were norovirus. That means cleaning public spaces more frequently and making sure employees are symptom-free for several days before returning to work.

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