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External factors, forces, and agents play a significant role in the causation of human disease. The environment in which we live is hardly benign and we are constantly exposed to its potential hazards. These include physical, chemical, thermal, and electrical forces as well as ionizing radiation. Noncommensal microbiological organisms are also external agents. While within the spectrum of human disease there are conditions where external factors have little or no role, such as some inherited disorders of metabolism or the muscular dystrophies, in most human afflictions external factors interact with host factors to produce disease. Certain lung cancers are linked to cigarette smoking and liver disease is a well-documented consequence of heavy alcohol consumption, yet in spite of the clear evidence that links these two agents to these diseases, it is also the case that only a minority of smokers and drinkers respectively develop them. At the other end of the spectrum lie the instances where external forces or agents are essentially the only factors as in trauma from a natural catastrophe like an earthquake or a human-made event such as a motor vehicle crash.
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The environmental hazards any one individual is exposed to are determined by a variety of factors including geography, socioeconomic status, and culture. This is true not only for infectious entities such as malaria which depends on an insect vector that itself requires certain climatic conditions for its existence but also for various forms of trauma. Cobra bites are a common problem in India but not in the United States. Motor vehicle-related injuries, gunshot wounds, cutting injuries, and disorders related to contaminated air and water vary from country to country and regionally within countries just as do the incidence of infectious and other diseases.
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This chapter will deal with diseases or conditions that are primarily the result of external factors but will not discuss diseases caused by microorganisms except to mention that infections, often from normal flora, are a common complication of trauma and remind readers that many vectors that spread infection require the physical trauma of a bite to inoculate the host. Tetanus caused by a toxin produced by the bacillus, Clostridium tetani, usually occurs after the introduction of that soil-dwelling organism into tissues as a consequence of a wound and similarly gas gangrene from Clostridium perfringens and other species. In the days before vaccination for tetanus, antiseptic surgery, and antibiotics, these entities were a common often-fatal complication of agricultural, martial, and other trauma and in some areas of the world today remain so.
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A measure of the burden of injury to the health-care system in the United States is the fact that 20% of all emergency department (ED) visits in 2015, some 41.6 million, were injury related. Injury is the leading cause of death for people ages 1–44 years in the United States, and because these deaths are in ...