Answer:
Fur trade
Pioneer settlers were sometimes pulled west because they wanted to make a better living. Others received letters from friends or family members who had moved west. These letters often told about a good life on the frontier. The biggest factor that pulled pioneers west was the opportunity to buy land.
Originally Spain, Great Britain, Russia, and the United States claimed the territory. In 1819, under terms of the Transcontinental Treaty, Spain ceded its claims to the territory to the United States.
John Jay's Treaty, 1794–95. On November 19, 1794 representatives of the United States and Great Britain signed Jay's Treaty, which sought to settle outstanding issues between the two countries that had been left unresolved since American independence.
Back then , the city was shocked by smallpox epidemic. Since smallpox is easily transmitted by contact, overcrowded areas are the perfect breeding spot for smallpox ( they still has not developed small pox vaccine back then)