39. A Fatal Bacterium
Modern research and scholarship have pinned the Black Death on Yersinia pestis, a bacterium with no spores. According to genetic analysis, a strain of Yersinia pestis that emerged during the Black Death caused that plague. However, it did not die off at the end of that period. Instead, it has lingered around ever since, mutating and reemerging periodically to cause further illnesses and plague outbreaks.
The most recent major outbreak, known as the Modern Plague or the Third Pandemic, erupted in China in the mid-nineteenth century. It was carried by rats aboard steamships all over the world, and claimed an estimated 10 million people.