Pathogens are microorganisms that cause disease.
Associated with sewage and biosolids, pathogens of importance include bacteria, viruses, protozoa, and helminths. In a non-scientific context, pathogens are often called germs. 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.