 One of the most notorious diseases in history is confirmed in the U.S. The plague killed as many as 50 million people in Europe in the 14th century. It's lived on as part of culture, but it remains a real, although rare, illness. One of those rare cases was confirmed in Pueblo County, Colorado. That's in the southeast part of the state, which is within the larger region of the U.S. where plague is most common, although there are only an average of seven cases in the U.S. per year.