A collection of scanning electron micrographs of untreated water from a wild stream, revealing a myriad of bacteria, protozoa, and algae. 

Many of the organisms in untreated waters are harmless, but the presence of those that are pathogenic, such as Giardia and virulent strains of E. coli make drinking water from natural streams typically unsafe to both humans and other animals. Furthermore, in our modern age, it isn’t uncommon to find toxic chemicals and debris present among the natural infusoria of freshwater streams.

Courtesy: CDC Public Health Image Library