Contingent contagion

Contingent contagionism was a concept in 19th-century medical writing and epidemiology before the germ theory, used as a qualified way of rejecting the application of the term "contagious disease" for a particular infection. For example, it could be stated that cholera, or typhus, was not contagious in a "healthy atmosphere", but might be contagious in an "impure atmosphere". Contingent contagionism covered a wide range of views between "contagionist", and "anti-contagionist" such as held by supporters of the miasma theory.

A form of contingent contagionism was standard in medieval European medicine. Contagion was not conceptualised as restricted to physical contact. A corruption of air could be transmitted from person to person, at short range.

By the 1840s public health policy, at least in the United Kingdom, had become a battleground between contagionist and anti-contagionist parties. The former, in particular, supported quarantine measures against epidemics (such as the cholera pandemic). The latter opposed quarantines. Anticontagionists, for example, argued that infection could be at a distance, from a cause that could be sporadic and possible diffused through the air, and taking advantage of "predisposed" individuals. Public health measures quite typically combined contagionist and anti-contagionist aspects. Anti-contagionists, such as Florence Nightingale who was a convinced miasmatist, could collaborate with contingent contagionists on sanitary measures.

Decomposing organic waste, as "", was considered implicated in many diseases, because of the gases it generated. The application of contingent contagionism could be that there was a contagious agent that was spread by filthy conditions. Sanitation as cleaning was therefore directly associated with public health. It has been commented that those involved in public health at this time, successful in bringing down death rates, "often attributed disease causation to levels farther up the causal chain than direct biological mechanisms".

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