Learn how knowledge about tuberculosis has changed through time, in this video adapted from AMERICAN EXPERIENCE: The Forgotten Plague: Tuberculosis in America. In the early 19th century, tuberculosis (then known as consumption) was widespread and a common cause of death. However, people did not understand what caused the disease. In 1882, Robert Koch, a German scientist, was the first to isolate tubercle bacillus–the bacteria that causes tuberculosis–and explain how the disease was spread. By the end of the century, the germ theory of disease was accepted by the medical community and “consumption” became known as tuberculosis. Robert Koch was awarded a Nobel Prize for his discovery.