We also fear animals that can kill us: sharks, poisonous snakes, poisonous spiders, bears, packs of wolves, lions, tigers, lynx, pirannahs, and the like. All of these animals combined kill tens or hundreds of people a year in the entire United States and aren't a major killer internationally either.
Diseases, which we have less of an acute fear of, are far more deadly than either disasters or killer animals. Relatively minor diseases (in the U.S.) like tuberculosis (TB) or meningitis or kidney infections each kill more Americans year in and year out than killer animals do. And, while this year hurricanes will kill more than 1,000 people (which is more than the diseases listed above), more years and on average, disasters kill far fewer people each year.
The worst disaster in the entire known history of humanity (and likely, of all time) was the flood of the Huang He River in China (also known as the Yellow River) in 1931 which killed 3.7 million people.
By way of comparison, the Spanish Flu in 1918 killed 25-50 million people. The bubonic plague killed about a quarter of the people in the Mediterranian in 542 CE, killed 25 million people in what is now France in 588 CE, killed 200 million people in the mid-14th century in Eurasia (a third of the population compared to a mortality rate of about 1% for people in New Orleans during Hurricane Katrina), and killed about 12 million people in India and China in 1855. AIDS killed 3 million people in 2004 alone. Tuberculosis kills 2 million people every year, and malaria kills 1 million people every year.
European diseases did more to destroy the Native American population of North America than any other factor. For example:
In 1698, St. Cosme, a Jesuit missionary traveling down the Mississippi River, recorded the devastating effects of smallpox among the Qawpaw: "We were deeply afflicted by finding this nation of the Arkansas, formerly so numerous, entirely destroyed by disease. Not a month has elapsed since they had rid themselves of smallpox, which has carried off most of them. In the villages are now nothing but graves ... and we estimated that not a hundred men were left. All of the children had died, and a great many women."
In the Lower Mississippi Valley (between Memphis, Tennessee, and Natchez, Mississippi) there were at least 50 towns between 1450 and 1550. By 1600, that number had dropped to ten--a decline of 80 percent in only 50 years.
Between 1698 and 1725, native populations living within the borders of French Louisiana experienced at least 13 epidemics, roughly one epidemic every two years. All epidemics resulted in some magnitude of native population loss, and the number of named groups in the colony dropped from 170 to 15, a 91 percent decline.
While some Native American rights activists cite these epidemics as European caused genocide, they were an almost inevitable consequence of European-Native American contact in an era when medical science was not very advanced. "All that was required
to produce an epidemic was one mildly sick European who happened
to come into contact with one healthy American Indian." The best guess that exists today for why the Native American population was devistated by European diseases, rather than the other way around is:
[T]he relative lack of domesticated herd animals in America. Herd animals are our richest source of disease microorganisms; we share in common with pigs and other barnyard animals diseases such as influenza. The fact that Native Americans had few domestic animals—dogs, llamas, guineas pigs and couple species of fowl—meant that they had relatively few immunities compared to those Europeans gained from their extensive interactions with dogs, barnyard fowl, horses, donkeys, pigs, cattle, goats, and sheep.
(This doesn't mean that Europeans dealt honorably with Native Americans, but it does dampen the claims of genocide. Blaming Europeans for stealing Native American land and forcibly relocating Native Americans, morally culpable acts all, is much better supported by the historical record, than the claim that the Europeans tried to bring about the mass death that was caused by diseases spread upon their arrival in North America).
Bird flu has killed 52 people as of this May, but could easily kill millions, the Spanish Flu of 1918 was a form of bird flu. Or, the disease could be any of hundreds of others out there waiting for the mutation thate suddenly makes a deadly disease far more contageous.
Modern science has given us the tools to understand what is going on much better than our predeceassors, but has also made the world a much smaller place, allowing diseases like West Nile virus to spread worldwide very quickly through people and goods travelling by air from country to country. Diseases that might once have been confined to small rural villages like those in Niger, can now be spread by an aid worker, government official, soldier, job seeker or farmer bringing goods to market to a major city and from their to someone who may travel to anyplace else in the world -- perhaps Paris, perhaps Atlanta, perhaps Lagos.
We know from experience that the species can survive even a massive outbreak that kills 30-40% of the population, but we'd prefer not to go through that experience again.