Avian flu.............

"But the avian flu may not remain trivial for long. The virus — known as H5N1 — changes very quickly. Humans have no natural immunity to it. And infected birds may be able to transit it before they show symptoms. Any virus with these characteristics could devastate the human population while causing massive economic and social chaos.
And, indeed, they have: Influenza viruses like avian flu have caused the greatest pandemics in world history. They mutate continually, making it almost impossible to wipe them out entirely. Even today in the United States, the flu kills over 30,000 people each year. But past epidemics have been much worse. Between 1918 and 1919, the Spanish Flu killed 40 million people around the world. Most were children and young adults. Given the world has nearly four times more people and far more efficient means of travel, another pandemic could spread more quickly and kill more people than before. If it were to become transmissible between humans, avian flu might kill 160 million to 200 million people."