A virus.. Nature's ultimate killer.. So lethal no other means can be ever more dangerous..
Though it is a matter of debate ever a virus is a living or non-living organism, there is no doubt of its reputation. Ebola, Spanish Flu, The Black Plague, Avian Flu, and now Swine Flu.
A virus is able to swap genes and RNA codes with its host and other viruses. This alone is its most powerful weapon. A killer that is able to adapt to its environment and spread at maximum rate by only choosing the genes it needs.
We mite think we are safe because Mexico is so far away. But with today's transportation system, the world has been shrunk to a puny metropolis. If a pandemic is to follow, nothing can stop it.
And so I pray it does not happen. Because mankind is still ill-prepared to deal with 'the next big one'. We are still busy fighting amongst ourselves to notice that we could be at the start of a total wipe out.