Smallpox was a contagious disease, brought from Europe to the Western Hemisphere, which killed thousands of Native Americans.
When the Europeans arrived to America, the New Continent, they brought with them war, slavery but also disease to numerous indigenous peoples.
The smallpox virus spread very easily in densely populated specially in Tenochtitláan.
The smallpox epidemic occurred in 1837 to 1838 along the Upper Missouri River, for instance, some Blackfoot bands suffered heavy losses, while neighboring Gros Ventre people flee nearly unscathed.