On October 8, 1871, flames sparked in the Chicago barn of
Patrick and Catherine O’Leary, igniting a two-day blaze that
killed between 200 and 300 people, destroyed 17,450 buildings,
left 100,000 homeless and caused an estimated $200 million
(in 1871 dollars; roughly $4 billion in 2021 dollars) in damages.

Legend has it that a cow kicked over a lantern in the O’Leary
barn and started the fire, but other theories hold that humans
or even a comet may have been responsible for the event that
left four square miles of the Windy City, including its business
district, in ruins. Dry weather and an abundance of wooden
buildings, streets and sidewalks made Chicago vulnerable to
fire.

The city averaged two fires per day in 1870; there were 20 fires throughout Chicago the week before the Great Fire of 1871.

Despite the fire’s devastation, much of Chicago’s physical
infrastructure, including its water, sewage and transportation
systems, remained intact.