On Oct. 8, the fire ignited on the barn behind the house of Patrick and Mary O'Leary, who lived at 13 DeKoven Street. The firemen at first went to the wrong location, and by the time they got to the fire, its flames had already rapidly spread toward the east and the north. Because much of the city's infrastructure was constructed of dry wood, the raging blaze burned most of the commercial buildings, streets, industrial buildings and homes in its path.
The fire was too massive and intense for firemen to contain, and the blaze continued to spread and rage out of control for two days. By the time heavy rainfall helped extinguish the fire on Oct. 10, the fire had burned about 200,000 acres of land, destroyed the entire Chicago business district, leveled approximately 18,000 structures and caused $200 million in property damage -- about $3.7 billion in today's dollars. This figure represented approximately one-third of the city's total value in 1871.
Because most homes were made of wood, the fire consumed many small houses and enormous mansions in most areas of the city. Many apartment-style complexes, which housed numerous families, were destroyed. About 100,000 people -- one-third of the city's total population -- were left homeless.
Because the fire occurred after 9 p.m., many people were sleeping, unprepared and vulnerable when the blaze initially spread. Alert systems available at the time were not as technologically advanced as they are today. As a result, the raging fire caused more than 300 deaths.