How worried should you be about swine flu?

That’s the question of the hour for world health officials and the rest of us alike.

For decades, virologists have worried about exactly this nightmare scenario: a brand new influenza virus makes the leap from birds or pigs to human beings and then begins to spread freely from person to person. The imaginary nightmare would be even scarier if it began in one of the world’s densely-crowded megacities–a place like Mexico City, for example–where it could infect millions of people before anyone knew it even existed.

That’s exactly the sort of nightmare that seemed to be unfolding as swine flu emerged and began to spread.