There were several reasons the Comanches and Kiowas had been angry in 1852. The first was they had recently been devastated by epidemics of smallpox and cholera. Their first experience with smallpox had been an epidemic (1780–81) so severe that it caused the disappearance of some Comanche divisions. The Comanche were hit again by smallpox during the winter of 1816–17. The wave of immigration from the California Gold Rush first brought smallpox (1848) and then cholera (1849) to the Great Plains. These were devastating to every plains tribe, but especially to the Comanches and Kiowa. The government census estimated a drop in the Comanches' 1849 population of 20,000 to 12,000 by 1851, and the Comanches never recovered from this loss. Smallpox struck again from New Mexico during 1862 and is believed to have been equally devastating. Cholera returned in 1867. By 1870, the Comanches numbered less than 8,000, and their numbers were still dropping rapidly.