Well, DUH! What Could Have Caused This "Mystery Illness"?!?

This is no "mystery" to me -- it sounds like Norovirus, which is contagious.  Do the math: 400 people + unsanitary conditions + no common sense = illness.  And the explanation is below.

Fox News: Mystery as huge group of people falls violently ill while hiking through remote part of Grand Canyon

"We [left] early morning on the 6th and then set up camp. We had a wonderful campsite," Griffiths said. "I was throwing up, just a lot of GI issues and then it just progressively got worse and worse."

 "Another friend in her group also got severely ill, and the group ended up hiking back several miles to a village in the canyon to get medical attention. 

"He discovered our temperatures were really high. Mine was 104 and my friend was 105. We were violently ill. They were able to admit us to the clinic there because they considered it life threatening," Griffiths told the outlet."

..........

"When they got home, the Blair family saw that over 300 people had posted about similar experiences on Facebook, with symptoms of vomiting and diarrhea. 

"It's unclear what led to the mass illness, but Blair and her family - who are experienced hikers - told Fox 10 that nearly 400 people were camping in close quarters. Sanitary bathrooms were difficult to find, she said."

"There is an excessive heat warning at this time," the tourism authority wrote. "The Havasupai Tribe reminds all tourists to properly hydrate before, during and after your visit to Havasupai. Please consider supplementing your water intake with electrolytes." "Lastly, it is critically important that all tourists take our their trash, camping supplies and personal items," the post continued. 

"Properly dispose of hygiene items. The trash bins in the composting toilets are not for your personal campsite trash... The cost to remove tourist trash and gear by helicopter is substantial."