Park officials are investigating a cluster of a mystery illness striking rafters and leaving them with excruciatingly painful infections. 

The National Park Service (NPS) confirmed earlier this week that it has received multiple reports of illnesses among people who rafted along the Colorado River, which flows through a 277-mile stretch at the bottom of the Grand Canyon, over the past several months. 

The Grand Canyon is located in Arizona, but the Colorado River as a whole spans 1,450 miles, running through Wyoming, Colorado, Utah, New Mexico, Arizona, Nevada and California. 

The exact virus behind the reports has not been confirmed in laboratory testing or by NPS, but rafters have reported flu-like symptoms such as fatigue and fever, along with infections from seemingly benign injuries. 

Matthew Wappett, from Utah, wrote on social media that he has been sick since rafting the river several weeks ago. 

Wappett said he entered the water in mid-May and finished his journey on June 2. Upon returning to land, he noticed his knee seemed scraped. 

Just three days later, he rushed to the emergency room, as his knee had become severely swollen and he was suffering a fever and 'bone-crushing' joint pain. 

He said doctors initially suspected a staph infection - caused by Staphylococcus bacteria that can live on the skin and enter the bloodstream through an open wound - in his knee and started him on antibiotics, though there was not a positive staph test. 

Though antibiotics reduced the swelling, Wappett wrote that he 'continued to have fevers, severe bone and joint aches' and was also diagnosed with pneumonia. 

'Essentially, I've felt like crap ever since returning home... and no, it's not the post-trip blues,' he wrote in a Grand Canyon rafting Facebook group.  

Wappett also told 12News KPNX that he feels 'horrible,' adding that 'it feels like I did a hard workout every day, even though I've done nothing except for sit on my butt.' 

He is still waiting on additional test results looking for signs of mosquito-borne illnesses such as dengue fever and fungal diseases like Valley Fever. He also said on Facebook that he has been contacted by NPS epidemiologists.

'Not sure what this is, but I wouldn't wish this misery on my worst enemy...it's really taken a toll on me physically and mentally. Please be careful out there!' he wrote. 

NPS said in a statement this week that it 'is aware of the specific river trip and the reported illnesses described in numerous Facebook posts on the Grand Canyon Rafting community site.

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

'At this time, the investigation is ongoing, and we are not able to comment on the extent of the illnesses, potential diagnoses, or other details while the investigation is underway. We will share additional information with the public as it becomes available.'

NPS has not released any information regarding the number of cases. 

One user in the same Facebook group as Wappett wrote on July 2 that he spoke with a friend who works as an epidemiologist about the mystery illness. 

He claims the epidemiologist said that based on the symptoms and reports of potential mosquito bites, 'it's sounding viral - and symptoms are a good match for both Dengue and Chikungunya.' 

The user also claims the epidemiologist noted Valley fever 'should also be explored as a possibility.'

Other users in the group have speculated about Chikungunya and Legionnaires' disease.

Dengue is present in more than 100 countries globally and is a year-round threat to travelers and local residents in high-risk countries. While most cases occur in international travelers, about 100 US locally acquired cases are reported each year. 

Spread when a person is bitten by an infected Aedes aegypti mosquito, it often causes an asymptomatic infection, but in some patients it can trigger life-threatening internal bleeding, respiratory distress and heart failure. It can also lead to shock and organ failure, especially of the liver, brain and heart.

Patients may also develop dengue shock syndrome, a condition where severe bleeding leads to a rapid drop in blood pressure and causes the body to go into shock.

Chikungunya, meanwhile, has been the subject of several recent CDC travel warnings for countries like Bangladesh and Sri Lanka. 

The virus is transmitted through the bite of infected Aedes mosquitoes, the same species that spreads dengue and Zika. Infection brings on a series of debilitating symptoms, including high fever and severe joint pain. Headaches, muscle aches, swelling and rashes are also common.

While most acute symptoms subside within a week, a portion of sufferers experience persistent and painful arthritis that can linger for months or even years. 

Like Dengue, chikungunya is mostly associated with travel, but locally transmitted cases were reported in 2025 in New York, Florida and Texas.

Valley fever is not a mosquito-borne illness but instead is a fungal infection caused by breathing in Coccidiodes spores from disturbed soil, largely in Arizona, where the Grand Canyon is, and California's Central Valley. 

Symptoms such as fever, headache, cough, chest pain and fatigue usually set in one to three weeks after inhaling the spores. 

And Legionnaires' disease is a severe form of pneumonia spread through contaminated water vapor.

Infected patients initially suffer from a headache, muscle aches and fever, before the disease also triggers a cough, shortness of breath, chest pain, nausea, vomiting, confusion or other symptoms.

In severe cases, patients can suffer from severe pneumonia and the potentially fatal complication sepsis, when the bacteria spreads to the blood.

There has also been speculation about West Nile virus, the leading cause of mosquito-borne illness in the US, resulting in 2,000 locally transmitted cases per year. The majority of US cases occur in Arizona, California, Colorado and Texas.