But far from being a harmless source of fertilizer, dog feces is both an environmental contaminant (and is classified as such by the Environmental Protection Agency) and a source of pathogens that can infect people. Like human excreta, dog poo teems with pathogenic microbes, such as strains of E. coli, roundworms, and other parasites. One of the most common parasitic infections in Americans is the result of their exposure to dog feces. The dog roundworm Toxocara canis is common in dogs and, because of the ubiquity of dog feces, widespread in the environment. It can contaminate soil and water for years.