There are many luxuries that we take for granted that are rather recent developments in our history. One such luxury is avoiding what was known as milk sickness. During the 19th century, milk sickness claimed the lives of countless thousands and it did so in a very violent manner, often manifesting in serious tremors, vomiting, coma and eventually death. It took a long time for early settlers to figure out what was causing milk sickness. We owe it to a woman named Dr. Anna Pierce Hobbs Bixby along with the rumored input of a Shawnee woman for getting to the root of the mystery.
As it turns out, milk sickness is caused from consuming the meat or milk of cattle that have fed on white snakeroot (Ageratina altissima). Once belonging to the same genus as boneset and Joe Pye weed, white snakeroot loves growing in the same kinds of areas that settlers grazed their cattle. It contains a toxin known as tremetol that, when consumed by cattle, builds up in the meat and milk. If cattle products are then ingested by humans, milk sickness is soon to follow. Famously, Abraham Lincoln's mother is said to have been killed by milk sickness.
White snakeroot blooms in late summer and continues through fall. In some areas it is the dominant species growing, which is great if you are one of the many pollinators that utilize this plant. Thankfully, today milk sickness is an almost unheard of occurrence and we owe much of that to Dr. Bixby.
Further Reading: http://plants.usda.gov/core/profile?symbol=agal5