A pathogen is an agent that causes disease. Pathology is the scientific study of human disease. One could argue that anything that causes disease is therefore by definition a "pathogen." Sunlight is the environmental agent that (with excessive exposure) induces the potentially fatal skin cancer known as melanoma. Ordinarily, however, most people do not consider sunlight to be a pathogen. An unbalanced diet may result in nutritional deficiencies which can lead to diseases such as pellagra (caused by a niacin deficiency) and scurvy (caused by inadequate vitamin C in the diet). Nevertheless, the failure to consume a balanced diet is not considered to be a pathogen. Generally, most students of disease refer to biological agents when they use the term pathogen. Such agents, which include viruses, bacteria, fungi , protozoa, and worms, cause a tremendous diversity of diseases.
Viruses, while considered biological agents, are not cellular organisms and accordingly, are not living in the usual sense. They are tiny particles consisting of either DNA or RNA as the genetic material and a protein coat, and they are incapable of metabolism outside of a living cell. Pathogenic viruses cause diseases of the respiratory system such as colds, laryngitis, croup, and influenza. Skin eruptions such as measles, rubella, chicken pox, and foot-and-mouth disease are viral in origin. The long list of viral diseases include insectborne Western equine encephalitis and yellow fever. Recently, some human cancers have been thought to be associated with viruses, perhaps because animal cancers such as the Lucké renal carcinoma and mouse mammary carcinoma are known to be caused by viruses. One such human cancer is Burkitt's lymphoma, a childhood malignancy occurring primarily in Africa, which is associated with a herpes virus . AIDS results from infection with HIV and is a pandemic viral disease.
Bacteria are true cells but their genetic material, DNA, is not packaged in a nucleus as in all higher forms of life. Not all bacteria are pathogens, but many well-known diseases are caused by bacterial infections. Tuberculosis, cholera , plague , gonorrhea, syphilis, rheumatic fever, typhus, and typhoid fever are some of the very serious diseases caused by pathogenic bacteria.
Probably 100 million cases of malaria occur each year primarily in Africa, Asia, and Central and South America. Malaria is caused by four species of the protozoan Plasmodium. Amebiasis and giardiasis are parasitic protozoa infections. Protozoa are single-celled animals. Thrush (candidiasis ) is a common infection of mucous membranes with a yeast-like fungus. Valley fever (coccidioidomycosis ) is a fungal infection generally limited to the lungs. Athlete's foot is a common skin infection caused by a fungus.
Worms that infect humans are a significant health problem in those parts of the world where there is inadequate public health protection. Examples of helminth infections are the beef and pork tapeworms. The consumption of inadequately cooked pork can result in trichinosis, which is a roundworm (nematode) infection of human muscle. Trematode flukes cause an extraordinarily important infection in Asia and Africa called schistosomiasis with hundreds of millions of individuals infected. Perhaps the most common parasitic helminth infection in the United States is enterobiasis known as pinworm or seatworm infection, which is a common condition of children with improper personal hygiene.
Many diseases caused by microbial pathogens can be treated with a diversity of antibiotics and other drugs. However, viral pathogens remain intractably difficult to manage with drugs.
[Robert G. McKinnell ]
path·o·gen / ˈpa[unvoicedth]əjən; -ˌjen/ • n. Med. a bacterium, virus, or other microorganism that can cause disease. DERIVATIVES: path·o·gen·ic / ˌpa[unvoicedth]əˈjenik/ adj. path·o·ge·nic·i·ty / ˌpa[unvoicedth]əjəˌnisitē/ n. pa·thog·e·nous / pəˈ[unvoicedth]äjənəs/ adj.