Originally published on February 12, 2015.  Viruses take the cake as tiniest of the waterborne disease-causing microorganisms—smaller than both protozoa and bacteria. These nasty little bugs are also the least understood by scientists, and cause the greatest range of symptoms across infected individuals. The good new is, in North American backcountries, viruses are typically considered much less of a concern than the other pathogenic threats.

Bacteria are everywhere—on you, in you, in the soil, and yes, even in the wilderness’ cool, refreshing water sources. In fact harmless species of these single-cell organisms exist naturally in the backcountry’s rivers and pools. But humans and animals can carry harmful bacteria as well, and spread these pathogens to the water, making it risky if you happen to drink from the wrong place at the wrong time. Some of these bacteria are the same notorious headline grabbers associated with foodborne outbreaks or epidemics after natural disasters. We’ll discuss those and others, but first a few general facts.