From Wikipedia:

The Great Stink or The Big Stink was a time in the summer of 1858 during which the smell of untreated sewage almost overwhelmed people in central London, England.

Part of the problem was due to the introduction of more modern flush toilets. While these were a step forward on the chamber-pots that most Londoners used, they dramatically increased the volume of water and waste that was now poured into existing cesspits. These often overflowed into street drains originally designed to cope with rainwater, but now also used to carry outfalls from factories, slaughterhouses and other activities, contaminating the city before emptying into the River Thames.

Cholera became widespread during the 1840s (not least because many people believed the disease was due to air-borne “miasma”; no one then realised that the disease was water-borne — that discovery was not made until 1854 by London physician Dr John Snow after an epidemic centred in Soho), and sanitation reform soon became a high priority. Bringing together several separate local bodies concerned with sewers, the consolidated Metropolitan Commission of Sewers was established in 1848; it surveyed London’s antiquated sewerage system and set about ridding the capital of an estimated 200,000 cesspits — an objective later accelerated by the “Great Stink”.

In 1858, the summer was unusually warm. The Thames and many of its urban tributaries were extremely polluted; the warm weather encouraged bacteria to thrive and the resulting smell was so overwhelming that it affected the work of the House of Commons (countermeasures included draping curtains soaked in chloride of lime, while members considered relocating upstream to Hampton Court) and the law courts (plans were made to evacuate to Oxford and St Albans). Heavy rain finally broke the hot and humid summer and the immediate crisis ended. However, a House of Commons select committee was appointed to report on the Stink and recommend how to put an end to the problem.