Since June 7, more than 200 workers and visitors to the Yellowstone and Grand Teton parks have reported gastrointestinal illness, and some have tested positive for norovirus.
The Centers for Disease Control and Prevention says the norovirus is the most common cause of acute gastrointestinal illness, and can cause symptoms of diarrhea, vomitting, fever and nausea. The virus infects some 21 million people each year.
Yellowstone spokesman Al Mash said most people contract the virus while camping when they fail to properly store their food and wash their hands with soap and water prior to eating.
"Don't rely on hand sanitizer. It's good for a while if you don't have access to water," he said. "But sanitizer is a poor second to washing your hands."
While it may be difficult to wash your hands while camping, most sporting goods stores sell soap slivers and biodegradable soap for use in the wildnerness, Mash added.
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