What's the most deadly killer virus in all of human history? The Ebola virus? AIDs? Smallpox? Maybe the Black Death?
Actually, the most lethal epidemic killer is more common than you might think, the flu.
As a deadly threat, the flu virus doesn't usually get much respect. While a bout of flu is no fun to suffer through, we certainly don't expect it to kill us. Unfortunately, just because one strain of flu seems relatively harmless doesn't mean the next one won't become a major epidemic killer. This is because the flu is never the same disease twice.
The flu virus's main trick is an extraordinarily fast rate of evolution, allowing it to do an end run around our bodies' best defenses. It works like this. Our bodies fight viruses by creating antibodies, which are tailor-made for whatever specific virus is attacking. These antibodies fit the shape of the virus exactly, locking on tight and destroying them.
Doctors can help this natural process by giving you a tailor-made vaccine for each specific virus. Unfortunately, the flu virus mutates so rapidly that each new season it's got a different shape. What made you merely sick one winter might slip past your defenses and kill you the next.
The worst flu epidemic, caused by an unusually sudden change in the virus's shape, occurred in 1918. It killed between twenty and forty million people worldwide. The strain could kill a person literally overnight, sometimes in a matter of hours.
Unfortunately, the flu virus keeps on mutating. It's anyone's guess when it will cross the line again from annoying inconvenience to deadly epidemic.