Cholera outbreaks and pandemics
That year, cholera was transmitted along the California, Mormon and Oregon Trails as 6,000 to 12,000 are believed to have died on their way to the California Gold Rush, Utah and Oregon in the cholera years of 1849–1855. It is believed more than 150,000 Americans died during the two pandemics between 1832 and 1849.
California Trail
These census numbers show a 363,000 population increase in the western states and territories between 1860 and 1870. Some of this increase is due to a high birth rate in the western states and territories, but most is due to emigrants moving from the east to the west and new immigration from Europe. Much of the increase in California and Oregon is due to emigration by ship, as there were fast and reasonable low-cost transportation via east and west coast steam ships and the Panama Railroad after 1855. The census numbers imply at least 200,000 emigrants (or more) used some variation of the California/Oregon/Mormon/Bozeman trail(s) to get to their new homes in the 1860–1870 decade.