According to the National Institutes of Health, disease-causing agents, also referred to as infectious agents, are any of a number of microbes that are capable of causing disease. classify disease-causing agents into six groups: bacteria, viruses, fungi, helminths, protozoa and prions.
According to MedlinePlus, Bacteria are the most diverse group of organisms on Earth, but fewer than one percent of known bacteria are harmful to humans. Streptococcus, Staphylococcus and E. coli are among the most recognizable disease-causing bacteria. Viruses are not organisms, and according to the National Institutes of Health, without a host cell, they are incapable of metabolizing or reproducing. Viruses are responsible for many human diseases, including the common cold, chicken pox, measles and HIV.
The National Institutes of Health explain that the primary role of fungi is to decompose organic material, and few species are associated with human disease. Ringworm, yeast infections, thrush and histoplasmosis are all caused by fungi. Helminths are very simple invertebrates. Some species are parasitic and cause diseases ranging from swimmer's itch to trichinosis. Protozoa infect humans with disease through contaminated food and water or bug bites. Protozoa are responsible for chronic diarrhoeal diseases and malaria.
The National Institutes of health point out that prions are the least understood of the infectious agents. They consist only of protein and are linked to central nervous system disorders that include Creutzfeldt-Jakob disease in humans and mad cow disease in cattle. Many prion diseases appear to be inherited, but others are linked to ingesting infected tissues.