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.