The Great Fire of London accidentally saved thousands of lives by killing the plague

The bubonic plague killed roughly 100,000 Londoners — about a quarter of the city's population — in 1665. The following year, the Great Fire of London burned for four days, destroying 13,000 houses and most of the medieval city. The dense, wooden rat-infested slums where plague had flourished were almost entirely destroyed. A new London, built largely of brick and stone with wider streets, replaced them. Historians regard the fire as one reason London never experienced a major plague epidemic again: the fire and the rebuilding it forced inadvertently redesigned the city's public health infrastructure.

Source: Tinniswood, A., By Permission of Heaven: The Story of the Great Fire of London (2003)