Miasma theory dominated medicine and delayed the real answer

For most of the nineteenth century, the reigning explanation for disease outbreaks was miasma theory — the belief that foul-smelling air itself carried illness, an idea with just enough surface plausibility (bad smells often did correlate with disease-prone slums) to survive for decades without real evidence.

Johnson shows how deeply this belief was embedded in institutions: sanitary reformers like Edwin Chadwick genuinely believed removing smells would remove disease, which led London to flush waste out of cesspools and into the Thames — the very river much of the city drank from, inadvertently making the water supply more dangerous while officials congratulated themselves on improving sanitation.

This is Johnson's setup for the whole book: an authoritative, well-intentioned, wrong idea, defended by respected professionals, standing directly in the way of a correct explanation that had actually already been proposed but lacked the evidence to overturn consensus.

Takeaway: a plausible-sounding wrong theory, backed by respectable institutions, can be more dangerous than an obviously bad one — it blocks the correct answer from even being seriously considered.