By the dawn of the 19th century, the deadliest killer in human history, tuberculosis, had killed one in seven of all the people who had ever lived. The disease struck America with a vengeance, ravaging communities and touching the lives of almost every family. The battle against the deadly bacteria had a profound and lasting impact on the country. It shaped medical and scientific pursuits, social habits, economic development, western expansion, and government policy. Yet both the disease and its impact are poorly understood: in the words of one writer, tuberculosis is our “forgotten plague.” During most of the 19th century, consumption, as tuberculosis was then called, was believed to be hereditary.