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.
Oregon 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 because of a high birth rate in the western states and territories, but most is from emigrants moving from the east to the west and new immigration from Europe. Much of the increase in California and Oregon is from emigration by ship, as there was fast and reasonably low cost transportation via east and west coast steamships 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 trails to get to their new homes between 1860 and 1870.