A disease is any condition which results in the disorder of a structure or function in a living organism that is not due to any external injury. The study of disease is called pathology, which includes the study of cause. Disease is often construed as a medical condition associated with specific symptoms and signs. It may be caused by external factors such as pathogens or by internal dysfunctions, particularly of the immune system, such as an immunodeficiency, or by a hypersensitivity, including allergies and autoimmunity.
The horseman on the white horse was clad in a showy and barbarous attire. [...] While his horse continued galloping, he was bending his bow in order to spread pestilence abroad. At his back swung the brass quiver filled with poisoned arrows, containing the germs of all diseases.
Vicente Blasco Ibáñez, The Four Horsemen of the Apocalypse (1916), (ch V).
Pesticides are used around the world but can persist in the environment, contaminating drinking and irrigation water with toxic organic compounds. One example is Vietnam where herbicides and dioxins applied during the Vietnam war entered the water cycle, possibly leading to cancers and abnormalities in newborn babies.
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