The 1837 Great Plains smallpox epidemic spanned 1836 through 1840, but reached its height after the spring of 1837 when an American Fur Company steamboat, the S.S. St. Peter, carried infected people and supplies into the Missouri Valley. More than 15,000 Native Americans died along the Missouri River alone, with some tribes becoming nearly extinct. Having witnessed the effects of the epidemic on the Mandan tribe, fur trader Francis Chardon wrote, "the small-pox had never been known in the civilized world, as it had been among the poor Mandans and other Indians. Only twenty-seven Mandans were left to tell the tale." The Commissioner of Indian Affairs in 1839 reported on the casualties: "No attempt has been made to count the victims, nor is it possible to reckon them in any of these tribes with accuracy; it is believed that if the above number (i.e., the number 17,200 for the upper Missouri River Indians) was doubled, the aggregate would not be too large for those who have fallen east of the Rocky Mountains."
Smallpox has afflicted Native Americans since it was carried to the western hemisphere by the Spanish conquerors, with credible accounts of epidemics dating back to at least 1515. The Mandan tribe, also called the People of the Pheasants, had previously experienced a major smallpox epidemic in 1780-81 which severely reduced their numbers down to less than a few thousand. Many other tribes along the Missouri river suffered smallpox epidemics during 1801-02 and 1831. Sporadic efforts were made to promote vaccination among the Native Americans since the turn of the nineteenth century, and a couple years after the Indian Removal Act the U.S. Congress took its first step in 1832 to generate public support for vaccination of the Native Americans. But shortly after passage of this congressional act to extend vaccinations to Indians, Secretary Cass stated that no effort would be made "under any circumstances" to send surgeons to vaccinate Indians up the Missouri River beyond the Arickaree tribe. This Great Plains epidemic spanned thousands of miles, reaching California, the northwestern coast and central Alaska before finally subsiding in 1840.
The S.S. St. Peter steamboat, traveling up the Missouri River to Fort Union from St. Louis, docked at Fort Clark near the two earth-lodge villages of the Mandan people on June 18, 1837. The disease spread to the Mandan people, and was of the most virulent, malignant hemorrhagic form. In July 1837, the Mandan numbered about 2,000; by October that number had dwindled to 23 or 27 survivors by some accounts, 138 by another account. On August 11, Francis Chardon, a trader at Fort Clark, wrote, "I Keep no a/c of the dead, as they die so fast it is impossible," and by the end of the month, "the Mandan are all cut off except twenty-three young and old men."
By the time the St. Peter made it to Fort Union, several deck hands had died, and Jacob Halsey, an American Fur Company clerk, showed visible signs of the disease. In an attempt to stop the spread of the disease, fort personnel performed primitive inoculations. Pus from Halsey's skin eruptions was used to inoculate approximately thirty Native American women and several white men living in or around the fort. Within two weeks, the women who received the inoculations began dying from the disease; eventually 27 of the 30 died.
As the disease reached a peak at Fort Union, the Assiniboines continued to arrive at the fort for trade. Halsey wrote, "I sent our interpreter to meet them on every occasion, who represented our situation to them and requested them to return immediately from whence they came however all our endeavors proved fruitless, I could not prevent them from camping round the Fort-they have caught the disease, notwithstanding I have never allowed an Indian to enter the Fort, or any communication between them & the Sick; but I presume the air was infected with it for a half mile..."
Later, a longboat was sent to Fort McKenzie via the Marias River. At Fort McKenzie the disease spread among the Blackfoot people housed there. The epidemic continued to spread into the Great Plains, killing many thousands between 1837 and 1840. In the end, it is estimated that two-thirds of the Blackfoot population died, along with half of the Assiniboines and Arikaras, a third of the Crows, and a quarter of the Pawnees. A trader at Fort Union reported "such a stench in the fort that it could be smelt at a distance of 300 yards", as the bodies were buried in large pits, or tossed into the river.
There have been many documented cases of smallpox being intentionally spread among the indigenous people of the Americas by colonizers. As observed by academic researchers in 1945, "Smallpox killed more Indians in the early centuries than did any other single disease. The historical record here recounted not only proves the truth of the above statement but also shows that this disease was the most dreaded of scourges, the most frequent disastrous dictator of destiny and action among the Indians. This fact the white man soon learned, for history records numerous instances of the French, the Spanish, the English, and later on the American, using smallpox as an ignoble means to an end. For smallpox was more feared by the Indian than the bullet: he could be exterminated and subjugated more easily and quickly by the death-bringing virus than by the weapons of the white man."
Devastated by epidemics, Native Americans grew increasingly suspicious of the colonists to the extent it became tradition that they had been purposely infected; trading became strained and hostilities and retribution increased. The white fur traders and settlers did their part during the 1800s to foster fear in the natives with threats and warnings that they would unleash the smallpox upon them. One such threat was delivered by fur trader James McDougall, who is quoted as saying to a gathering of local chiefs, "You know the smallpox. Listen: I am the smallpox chief. In this bottle I have it confined. All I have to do is to pull the cork, send it forth among you, and you are dead men. But this is for my enemies and not my friends." Likewise, another fur trader threatened Pawnee Indians that if they didn't agree to certain conditions, "he would let the smallpox out of a bottle and destroy them." The Reverend Isaac McCoy was quoted in his History of Baptist Indian Missions as saying that the white men had deliberately spread smallpox among the Indians of the southwest, including the Pawnee tribe, and the havoc it made was reported to General Clark and the Secretary of War. Artist and writer George Catlin observed that Native Americans were also suspicious of vaccination, "They see white men urging the operation so earnestly they decide that it must be some new mode or trick of the pale face by which they hope to gain some new advantage over them." So great was the distrust of the settlers that the Mandan chief Four Bears denounced the white man, whom he had previously treated as brothers, for deliberately bringing the disease to his people. After losing his wife and children to smallpox, and acquiring the affliction himself, he gave his final speech to the Arikara and Mandan tribes imploring them to "rise all together and not leave one of them alive", before dying on July 30, 1837.
Others who have written about intentional communication of smallpox to Native Americans during the 1836-40 epidemic include Ann F. Ramenofsky in 1987 and Ward Churchill in 1992. According to Ramenofsky, "Variola Major can be transmitted through contaminated articles such as clothing or blankets. In the nineteenth century, the U. S. Army sent contaminated blankets to Native Americans, especially Plains groups, to control the Indian problem." Churchill also asserted that in 1837 at Fort Clark the United States Army deliberately infected Mandan Indians by distributing blankets that had been exposed to smallpox, but he additionally alleged that the blankets were taken from a military infirmary in St. Louis, that smallpox vaccine was withheld from the Indians, and that an army doctor had advised the infected Indians to disperse, further spreading the disease and causing over 100,000 deaths. After an investigation of Churchill's writings and sources by the University of Colorado at Boulder, their Standing Committee concluded, "We do not find academic misconduct with respect to his general claim that the U.S. Army deliberately spread smallpox to Mandan Indians at Fort Clark in 1837, using infected blankets. Early accounts of what was said by Indians involved in that situation and certain native oral traditions provide some basis for that interpretation." Churchill was criticized, however, for not properly citing his more extreme details and not mentioning "native oral sources in any of his published essays about Fort Clark." Another frequently recounted story is that an Indian snuck aboard the St. Peter and stole a blanket from an infected passenger, thus starting the epidemic. The many variations of this account have also been criticized by both historians and contemporaries as fiction; a fabrication intended to assuage the guilt of white settlers. "The blanket affair was created afterward and is not to be credited", notes B. A. Mann.
While specific responsibility for the 1836-40 smallpox epidemic remains in question, scholars have asserted that the Great Plains epidemic was "started among the tribes of the upper Missouri River by failure to quarantine steam boats on the river", and Captain Pratt of the St. Peter "was guilty of contributing to the deaths of thousands of innocent people. The law calls his offense criminal negligence. Yet in light of all the deaths, the almost complete annihilation of the Mandans, and the terrible suffering the region endured, the label criminal negligence is benign, hardly befitting an action that had such horrendous consequences."