From Lakota America: A New History of Indigenous Power, by Pekka Hämäläinen (The Lamar Series in Western History; Yale U. Press, 2019), Kindle pp. 94-96:
The smallpox epidemic began in late 1775 in Quebec and spread down the eastern seaboard with the peripatetic British and Patriot armies. The epidemic’s most significant intrusion into the war came in 1781, when it caught up with the loyalist African Americans who had joined the British Army in a march across the South. In September General Charles Cornwallis was besieged in Yorktown, his black allies dying in masses and his soldiers succumbing to malaria. When Cornwallis surrendered, his army had nearly melted away under the double pathogenic assault. The first British Empire had come to an end.
While smallpox was thriving in the war-ravaged East, it found another opening some fifteen hundred miles to the west. This epidemic originated in Mexico City in August 1779 and moved from there to New Orleans, San Antonio, and Santa Fe by December 1780. There trade became the principal vehicle for transmission. Comanches, who dominated the lands amid those colonial capitals, were infected and seem to have passed on the pestilence to their trading partners, some of whom transmitted it into the Missouri Valley. Carried by equestrian Indians, the malady could travel far during its long incubation period, and a trading expedition may have reached the Missouri with the virus before succumbing to it.
Dying began in 1781—just as the British Army was wasting away at Yorktown—among Arikaras and Mandans. Lakotas contracted the disease around the same time, possibly while raiding. Oglala and Sicangu winter counts record two successive years of smallpox. They depict human figures in agony, their faces and torsos covered with red spots, documenting the infection’s aggressive spread from small blood vessels in the mouth and throat across the body until sharply raised, pus-filled blisters covered the skin; they capture the ineffectiveness of traditional healings methods in the face of an alien organism. There is no way of knowing how many died. Lakotas’ migratory way of life and dispersal into small hunting bands gave them a measure of protection against the pestilence, but cold and erratic weather around the outbreak must have compromised their ability to fight off the virus.
While the epidemic ravaged Lakotas, it nearly ruined the villagers. The virus found in the crowded villages an auspicious setting to spread. Arikaras may have lost more than three-fourths of their people, and they abandoned all but seven of their thirty-two villages. Mandan losses were similarly catastrophic. Their eight villages were reduced to two, and their thirteen clans became seven. The Hidatsa population was cut by half. An ancient political geography collapsed in a matter of months as the combined villager population of tens of thousands was reduced to roughly eight thousand. Thick clusters of Arikara, Mandan, and Hidatsa villages melted into thinly sprinkled nodes; permanent settlements no longer governed the riverscape. Cheyennes, too, were afflicted. Most of them abandoned their Missouri villages, making an abrupt and uncertain leap to a nomadic existence in the open plains to the west. Only one band, the Masikotas, stayed along the Missouri, attaching themselves to Lakotas.