They are very small organisms and are invisible to the unaided human eye. Bacteria are everywhere in our environment and life. Some bacteria are critically important to beneficial activities, some bacteria are neutral to humans, while some bacteria will cause human disease if a person is susceptible and comes in contact with a sufficient dose of the bacteria. A well-known bacterium is Salmonella found with chickens. Viruses are distinctly different from other pathogens because they are unable to reproduce outside the living cell they infect although they will remain virulent for a time.
Viruses are responsible for numerous human diseases including flu.
Helminths are usually in the form of tapeworms and roundworms, organisms frequently found in dogs, cats, and rats. Pathogens live in all healthy and ill humans, domestic animals, livestock, and wildlife, essentially all warm-blooded mammals. The germs are cast off by mammals through feces. Human feces are collected in sanitary sewers or septic tanks for disinfection and destruction, most often at centralized sewage treatment plants. Other germs may enter the treatment plant, lakes, streams, or groundwater through urban stormwater runoff.