The National Park Service has reportedly opened an investigation into a mysterious illness affecting multiple rafters who undertook separate trips through the Grand Canyon in recent weeks. The concern first surfaced when several people described a cluster of unexplained conditions on social media, saying their doctors have struggled to identify a cause. The matter is drawing attention because the trips were unconnected yet the affected parties are reporting similarly severe symptoms after returning home.

According to the article, the rafters set off on two-week journeys beginning as early as mid-May, with one group of 16 seeing four members fall ill after a mid-June trip. In a 1 July Facebook post to the "Grand Canyon Private Boaters" group, user Steven King listed symptoms including fever, chills, fatigue and pneumonia, with some suffering fluid on the lungs and loss of consciousness — one person briefly passed out at a doctor's office before being hospitalised. King also described a worsening "cobblestoning" skin condition on one person's shin, and said another began a course of rabies vaccine as a precaution. Online speculation about the cause has included mosquito-borne illnesses such as dengue fever and chikungunya, though officials have yet to determine any source. Paddling Magazine reported that park officials are leading the inquiry.