The discovery of disease-causing pathogens is an important activity in the field of medical science. Many viruses, bacteria, protozoa, fungi, helminthes and prions are identified as a confirmed or potential pathogen. In the United States, a Centers for Disease Control program, begun in 1995, identified over a hundred patients with life-threatening illnesses that were considered to be of an infectious cause, but that 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.