On this day in 1871, flames spark in the Chicago barn of Patrick and Catherine O'Leary, igniting a two-day blaze that kills between 200 and 300 people, destroys 17,450 buildings, leaves 100,000 homeless and causes an estimated $200 million (in 1871 dollars; $3 billion in 2007 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. Reconstruction efforts began quickly and spurred great economic development and population growth, as architects laid the foundation for a modern city featuring the world's first skyscrapers. At the time of the fire, Chicago's population was approximately 324,000; within nine years, there were 500,000 Chicagoans. By 1893, the city was a major economic and transportation hub with an estimated population of 1.5 million. That same year, Chicago was chosen to host the World's Columbian Exposition, a major tourist attraction visited by 27.5 million people, or approximately half the U.S. population at the time.
In 1997, the Chicago City Council exonerated Mrs. O'Leary and her cow. She turned into a recluse after the fire, and died in 1895.
Taken from: http://www.history.com/this-day-in-history [08.10.2012]