Marler told Business Insider that the idea he would have to warn people against drinking unfiltered, untreated water didn't cross his mind until 2018.
"Almost everything conceivable that can make you sick can be found in water," Marler said.
Unfiltered, untreated water — even from the cleanest streams — can contain animal feces, spreading Giardia, which includes symptoms such as vomiting and diarrhea and results in roughly 4,600 hospitalizations a year. Hepatitis A, which resulted in 20 deaths in a California outbreak in 2017, can be spread through water if it isn't treated. E. coli and cholera can also be transmitted via untreated water.