The leading cause of impaired surface waters in the united states is:

The leading cause of impaired surface waters in the United States is: Pathogens.
Pathogens (or germs) are what can produce diseases.

There are different types of pathogens: Bacterial (most bacteria are not harmful for the human kind, but pathogenic bacteria can cause infectious diseases); Viral (a few examples are influenza, or smallpox); Fungal (these usually consume dead organisms but some might be harmful for humans too); Prionic (these do not contain nucleic acids and cause diseases like, for example, the bad cow diseases); Algal (caused by a type of green alga known as Prototheca that lacks chlorophyll); and other parasites.