An outbreak from a yet-to-be-confirmed virus has infected at least 118 people visiting the Grand Canyon since May. It’s believed that norovirus is the root cause, though researchers are still looking for a definite answer. And while there are no reported deaths, helicopters have had to fly in for rescue missions.

Arizona hiker Kristi Key described one instance of a rescue in the Facebook group Grand Canyon Hikers. About four miles into Boucher Trail, she spotted four hikers who were on their last day out. Two had been vomiting “pretty violently the night before” while one other looked sick as well. The hikers said they didn’t need Key to call a rescue using her Garmin SOS (there’s no cell service), but after she headed back the group was in the same spot and still throwing up so she called in the rescue. She stayed with two of the hikers for hours until the helicopter arrived and picked up the sickest man while the rest finished the hike back out.

The one healthy hiker, Key noted, was throwing up in his room by the time he got back.

The rangers who came to help the hikers told Key that norovirus was “rampant” at the Grand Canyon Village, and there were several recent rescues on Boucher Trail for the same issues.

Reported cases do seem to be trending down, however, with most cases reported in May.

The chief of communications, partnerships, and external affairs at the Office of the Superintendent at Grand Canyon National Park, Jan Balsom, told the Daily Beast that the park hasn’t seen an outbreak of this kind in about 10 years.

Norovirus symptoms include vomiting, diarrhea, cramping, body aches, and fever, and it’s extremely contagious and spread through surfaces or contaminated food or drink. It’s the leading cause of illness from contaminated food in the United States. It is not, as you might imagine, something that you want to catch while hiking through remote parts of the Grand Canyon on a scorching hot Arizona summer day.

Though norovirus is the leading suspected cause by the state and federal public health team investigating the outbreak, documenting the true cause is harder to pinpoint in the Grand Canyon than, say, a city neighborhood. Norovirus testing is done through a stool sample that has a tight testing time window that river trips and long hikes usually outlast.

The National Park Service Office of Public Health told the Daily Beast that the investigation of the “heightened GI-illness” will “consider all potential sources. It is unknown at this time what the source of the illness is.” Trips along the Colorado River are the primary focus as of now. Boucher Trail, it should be noted, anecdotally appears to be a hotspot.

Park advisories ask that all visitors boil their water or use a chemical disinfectant (point-of-use filters don’t kill norovirus). People visiting the park should also avoid drinking from natural water sources like waterfalls, pools, and streams.