The fire began in a small barn owned by the O'Leary family in the southwest section of the city. No one is quite sure how the fire began. There are lots of other stories explaining the start of the fire including one about men gambling in the barn, someone stealing milk from the barn, and even one about a meteor shower. Conditions in Chicago were perfect for a large fire. There had been a long drought before the fire and the city was very dry. Most buildings were made out of wood, causing it too to expand.
Hopes this helps you!