The grave of Nancy Hanks Lincoln (1784–1818). Her body lies somewhere nearby, but we don’t know where. Spencer County, Indiana.

Pioneer people lived in terror of “milk sickness.” They could not figure out why you’d die if you drank some milk but not other milk. In some counties, half the population could die from it.

Nancy Hanks Lincoln’s farm was healthy, but her neighbors came down with the disease. Nancy was kind, smart, and active, always encouraging her children to read and improve their minds, and her instinct was to go help her friends despite the danger. She went to stay with them as they died. As she nursed them, she drank the milk in their house. She died, too. It was 1818, and her son Abraham was nine.

It took nearly a century for scientists to realize it was caused by cows eating wild white snakeroot—and that the cure was eating sugar, a rarity in Indiana at the time (the locals have certainly made up for that with their current diets).

Abraham Lincoln had suffered the first shocking death of a loved one, and at a critical age. The trauma shaped him to be the compassionate, literate, but emotionally detached man the country would later need him to be. All because a cow ate a weed.