In the 1700s, cities in Europe and North America were expanding quickly, and many people lived in tiny, cramped homes and tenement buildings. During this time, tuberculosis (TB) outbreaks were common--and deadly. Between 1700 and 1900, TB killed about 1 billion people around the world. Why? In the 1880s, a German doctor discovered that TB was caused by tiny germs that could spread whenever a person suffering from the illness coughed or sneezed--or...