Many of the graves uncovered appeared to hold entire families, who seemed to have died suddenly. Archeologists project that this was likely the result of pandemics of smallpox and other infectious illnesses brought by European immigrants to North America. These caused massive death tolls among Native American populations in 1780 and 1835, as they had no acquired immunity. Infectious diseases contracted from interactions with European fur traders are believed to have killed about 90 percent of the Indians living in the Northwest before European-American settlement of the area.