In 1918, an epidemic began that would kill more than 40 million people. A doctor wrote: "Dead bodies are stacked about the morgue like cordwood."

What was this terrible disease? It was a variety of the same influenza virus that causes "the flu" you catch again and again. How did this strain of a common virus become so deadly? And could that kind of deadly flu epidemic happen again?

The answers to those questions explain why we can't make a permanent vaccine against the flu, as we can against measles or smallpox. They also explain why public health officials worry so much about something you may have heard referred to as "bird flu." In this chapter, look for evolutionary processes that might help explain how new strains of influenza virus appear all the time. Then, solve the mystery.