GANGOTRI, India — High in the Himalayas, it’s easy to see why the Ganges River is considered sacred.

According to Hindu legend, the Milky Way became this earthly body of water to wash away humanity’s sins. As it drains out of a glacier here, rock silt dyes the ice-cold torrent an opaque gray, but biologically, the river is pristine — free of bacteria.

Then, long before it flows past any big cities, hospitals, factories or farms, its purity degrades. It becomes filled with a virulent type of bacteria, resistant to common antibiotics.

The Ganges is living proof that antibiotic-resistant bacteria are almost everywhere. The river offers powerful insight into the prevalence and spread of drug-resistant infections, one of the world’s most pressing public health problems. Its waters provide clues to how these pathogens find their way into our ecosystem.