Water filters and purifiers are used to clean water taken from a water source that harbors viruses and bacteria that are harmful to humans. Water filters screen out bacteria and protozoan cysts, such as Giardia, and water purifiers additionally remove or kill viruses,  like Hep A.  Giardia is the bug most commonly found in mountain streams and lakes.  Giardia comes from animal feces in the water.  Disease viruses, like Hepatitis A, can be contracted from waters in third world counties and come from human feces in the water.