A sewer line collapse in Maryland earlier this year spilled more than 360 Olympic-sized swimming pools’ worth of wastewater into the Potomac river just upstream of Washington, D.C. The incident may be the largest sewage spill in U.S. history,and it’s a quintessential example of fecal pollution, the most common source of sickness from natural waterways. Anytime we swim in a lake, river or ocean, we risk encountering waterborne pathogens.