Almost everyone survives the common flu because our immune systems are reasonably well prepared for it even without the latest vaccine. But a virus to which our body is completely naive could make most people very sick and a nasty one could kill 10 or 20 or even 40 percent of those who get it.

Wild birds also get the flu. They travel everywhere and infect birds living with humans--such as chickens in Asia. If an avian bug mutates to infect and easily spread among humans, we get an avian pandemic flu, i.e., a worldwide epidemic against which most of us have little resistance.

The odds of a virulent pandemic flu are very small. Pandemics occur a few times per century on average, but the only really dangerous recent one came in 1918 in the unique conditions of World War I. H5N1, the virus that to great acclaim has been killing thousands of birds and a few dozen humans, has circulated for nearly a decade with no evidence of mutating into a human-communicable virus.

But if we do get hit with pandemic avian flu, the consequences would be both extraordinary and unpredictable. No one knows how fast the flu would spread, how susceptible it would be either to existing drugs (such as Tamiflu, now being stockpiled around the world) or to quarantine, how quickly a vaccine could be put into play, or even what proportion of workers (including doctors and nurses) will show up for work as the bug spreads.

John E. Calfee is a resident scholar at AEI.