The discovery of disease-causing pathogens is an important activity in the field of medical science, as many viruses, bacteria, protozoa, fungi, helminthes and prions are identified as a confirmed or potential pathogen.

A Center for Disease Control program begun in 1995 identified over a hundred patients, with life threatening illnesses which were considered to be of an infectious cause, but could not be linked to a known pathogen. The association of pathogens with disease can be a complex and controversial process, in some cases requiring decades or even centuries to achieve.