Nancy Hanks Lincoln died of milk sickness on October 5, 1818, after nursing and comforting neighbors with the disease until she herself became ill. Feared by pioneers because its cause was unknown, milk sickness was caused when cows ate white snakeroot, a shade-loving plant found throughout the Ohio River Valley, and passed the poison to humans through their milk; the poison was not identified as coming from snakeroot until the early 20th century, and modern dairy practices have made the illness no longer a threat. The Pioneer Cemetery contains Mrs. Lincoln's gravesite along with other marked and unmarked graves of area residents from the 19th and early 20th centuries. For many years after the Lincolns left Indiana, Nancy Hanks Lincoln's grave was unmarked, until a permanent marker was erected in 1879 to honor the mother of the 16th president.