A mysterious illness has been plaquing a group of white water rafters who took a recent trip through the Grand Canyon, prompting an investigaion by the National Park Service.

Matthew Wappett, who rafted the Colorado River over a month ago, told NBC that he’s been sick for weeks.

“Oh, I’m horrible,” said Wappett. 

“I try to explain it to people. I mean, it feels like I did a hard workout every day, even though I’ve done nothing except for sit on my butt.”

The onset of the ailment began as a severe bacterial infection in Wappett’s knee. 

“My whole knee just swelled up,” he explained. “Then ended up on IV antibiotics and a full course of two different oral antibiotics to finally kill it.”

While the antibiotics nixed the infection in Wappett’s leg, flu-like symptoms have been lingering for weeks, and doctors are baffled. 

Other rafters describe similar symptoms in the Grand Canyon Private Boaters Facebook group, which they believe to be similar to chikungunya or dengue fever.

“The National Park Service is aware of the specific river trip and the reported illnesses described in numerous Facebook posts on the Grand Canyon Rafting community site,” said the National Parks Service in a statement to NBC. 

“The National Park Service Office of Public Health is leading the investigation in coordination with the appropriate public health partners.”

The investigation is ongoing and the National Park Service declined to comment on the extent of the illness or potential diagnoses. 

Wappett is waiting on lab tests to determine the source of his symptoms.

“I’m hoping to get some results back today or tomorrow,” Wappett said. “And, you know, I just take it day by day.”