Certainly there's cause for sober concern, if not alarm. For of the two qualities vital to a nasty pandemic—to spread readily and to be deadly—this flu, a brand-new strain of swine flu, or H1N1, seems to possess the first:

Evidence is strong that it spreads readily among humans. In that sense, it's an inversion of the bird flu.

Bird flu terrifies infectious disease experts because it kills about half the humans who get it—but it has so far failed to develop the ability to jump easily from person to person.

This swine flu, meanwhile, does seem to spread easily by airborne transmission. But how deadly is it?

Despite the 100-plus deaths in Mexico, we don't really know. And that's why epidemiologists are working frantically to figure out the Mexico mystery: Why do the death rates there appear to be so much higher than those in the United States?

In Mexico, it has reportedly killed about 100 of the 1,600 official suspected cases; elsewhere, it has appeared to take a far milder course, with