The first known smallpox epidemic to strike the native peoples of the coastal and interior Pacific Northwest arrived in the early 1770s, devastating large swathes of the population and causing significant demographic collapse – both from the disease itself and corresponding malnutrition from the deaths of hunters within tribes.
Poorly attested due to a lack of consistent European presence during the period, it most likely originated from Bruno de Heceta and Bodega y Quadra's 1775 expedition to the Northwest Coast, spreading as far inland as the Bitterroot Salish territories in western Montana and as far north as the Tlingit at Sitka.
Widespread pockmarks and scars among older native populations were recorded by European explorers for several decades afterwards, including Lewis and Clark and George Vancouver.
Brought to Santo Domingo in 1519, it spread to the mainland during the Spanish conquest of the Aztec Empire, causing widespread devastation across Mesoamerica.
Anthropologist Robert T. Boyd describes the epidemic as existing in a "shadowy period at the juncture of the protohistoric and historic eras", occurring almost immediately prior to sustained European presence in the Pacific Northwest.
Portlock, noting that the man had several surviving children under the age of 10–12 years, concluded that it had been spread by Spanish explorers some time in the 1770s.
[6] Sailors of the Columbia Rediviva noted the recent presence of smallpox among the Ditidaht of Vancouver Island and the Tillamook of the Oregon Coast.
[7] George Vancouver and Peter Puget, in their 1790s expedition to the Pacific Northwest, noted significant amount of smallpox scarring and partial blindness among Coast Salish groups, including the Twana.
[9] Christian missionaries and employees of the Hudson's Bay Company both reported signs of a past smallpox epidemic in the interior northwest during the mid-1800s.
However, the possibility of endemic smallpox in central Mexico, typical in such highly populated regions, would allow for disease transmission outside of periods of epidemic illness.
Support for the theory mainly stems from the earlier date for the pandemic estimated among the Tlingit and the presence of a Russian vessel in South Alaska during the Kamchatka epidemic.
Sweat baths and immersion in cold water, common among Plateau groups, could lead to the death of the infected by shock or pneumonia.