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 was 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.

Well, that nightmare is now unfolding hour by hour, from the epicenter of the pandemic in Mexico City to the furthest outposts of New Zealand and Eastern Europe. Mexico has shut down schools and is now asking all nonessential businesses to remain shuttered for a week. The latest report: Fort Worth, Texas has shut down its schools.

Given the nature of the highly infectious flu bug, there’s virtually no way to contain it, although precautions like closing school and cancelling big public events can slow its spread. That’s important, because it means less strain on the already beleaguered health care systems in many parts of the world. But no country or region or city can hope to prevent the virus from arriving. Chances are it’s already there.

For now, the biggest question is how virulent this new virus will be. Because it is new, none of us have immunity to it from previous exposures. Our immune systems have to recognize it, gear up our defenses, and eliminate the virus. In the meantime, it can make us pretty sick.

In people with compromised immune systems or who are already in poor health, bad flus can be deadly. The more virulent the strain, the more severe the symptoms and the deadlier it is likely to be.

The news from Mexico, where dozens of deaths have been confirmed, isn’t reassuring. But the relative mildness of cases elsewhere paints a more hopeful picture. No one really knows at the moment, so much of the world holds its breath, following the latest reports. (If you’re a flu tracker, one of the best places to stay abreast is the U.S. Centers for Disease Control’s swine flu information site.)

Keep this in mind: whatever the outcome of the next few weeks, researchers still won’t know exactly how this virus will behave when the next flu season rolls around in the fall. In previous pandemics, new viruses have caused mild cases when they first appeared, only to roar back with deadly force the following season.

So how worried should you be? Enough to be cautious. If cases have been reported in your area, it’s worth avoiding big public gatherings. Washing your hands is another good idea, since you can pick up the virus on surfaces and then contaminate yourself. Is it worth wearing a face mask? Although millions of people are wearing them in Mexico and elsewhere, health officials admit that they have no evidence to say whether or not masks can protect against swine flu.

Indeed, this newest epidemic reminds us that doctors and medical researchers don’t have all the answers. Science has made dramatic advances in understanding viruses and in developing vaccines to protect against them. But viruses remain a potent and unpredictable threat.

One thing is clear: when a vaccine to protect against the new swine flu is available, public health officials won’t have to spend much effort encouraging Americans to get a shot.

Tags: influenza, swine flu