Why Was the Plague So Deadly in 1665? — The Chain of Causes

London's conditions were perfect for plague transmission — By 1665, London had approximately 400,000 people crammed into an area designed for far fewer. Houses were overcrowded, sanitation was primitive (cesspits, open sewers), rubbish accumulated in streets, and rats were everywhere. The disease itself (Yersinia pestis, carried by fleas on rats) thrives in exactly these conditions. London's poorer outer parishes — St Giles in the Fields, Stepney, Whitechapel — were densest and suffered first and most severely. The physical environment made epidemic almost inevitable once plague arrived from the Continent via Dutch trading ships.

Nobody understood the actual cause — and wrong theories made things worse — The dominant explanation was miasma theory: that plague was spread by bad, corrupt air rising from rotting matter. This led to the lighting of bonfires (to purify the air), the burning of herbs, and the carrying of posies of flowers as personal protection. None of this worked. More damagingly, the order to kill 40,000 London dogs (thought to carry disease) removed the animals that naturally controlled rat populations — the actual carriers. Wrong medical theory actively worsened the death toll by removing a natural check on the rats that hosted the fleas that spread the disease.

Shutting up houses trapped the healthy with the sick — When a case of plague was identified in a household, the entire family was sealed inside for 40 days, with a watchman on the door. This was probably the most counter-productive measure of all: healthy family members who had not yet been infected were locked in with dying relatives and almost inevitably contracted the disease. Contemporary observers like Samuel Pepys noted that people went to great lengths to hide plague cases specifically to avoid being shut up — which meant the actual spread of infection was always worse than the Bills of Mortality recorded.

The rich fled, leaving the poor without medical help or resources — When plague became serious in summer 1665, Charles II and his court moved to Oxford. Parliament convened at Oxford. Most doctors, lawyers, and merchants left the city. This meant London's poor — who could not afford to leave — faced the epidemic with no access to medical attention, reduced trade, and abandoned local government. The social geography of the plague was stark: death rates were far higher in poorer parishes than in wealthier ones, partly because the poor could not flee and partly because overcrowding in poor districts facilitated transmission.

= A crisis that revealed the limits of Restoration government — The plague demonstrated that the Restoration government lacked the administrative capacity, medical knowledge, and social will to protect its poorest citizens. The king's flight to Oxford contrasted sharply with his personal presence fighting the Great Fire the following year — a distinction contemporaries noticed. The plague ended not through effective government action but because the winter cold killed off the flea population and the Great Fire of 1666 destroyed much of the overcrowded wooden housing that had harboured rats. Both endings were essentially accidental, not the result of policy.