Scientists use various models to evaluate risk, but researchers have long understood that ordinary people don't rely on mathematical probability to assess the dangers posed by some phenomenon - they use other kinds of rules.

For example, we're much more accepting of risks that we assume voluntarily. That's one reason why people who smoke a pack a day will go to a zoning board meeting to fight against a diesel bus yard.

People are also more accepting of risk when they have a sense of control. Driving a car is much more dangerous than riding in an airplane, but as an airplane passenger, you feel helpless. Another reason people worry more about airplane crashes than motor vehicle accidents is because the events are more noteworthy - a whole lot of people killed all at once, instead of one or two here and there, even though the latter adds up to a lot more people over time.

Then there is the question of familiarity. Whatever you may think about the safety of nuclear power versus coal, one reason that people were so averse to nuclear power was its novelty. We have been accustomed to accepting the risks of mining and burning coal for centuries, so we don't often think about them.

Some kinds of risks come with a moral stigma, which is why people drink bourbon and think their kids are doomed if they smoke pot.

But one of the most important factors is called the social amplification of risk - which is mostly the doing of the mass media. Plane crashes are major news stories, whereas most fatal motor vehicle crashes get only minimal attention from local media, if any. That's because plane crashes are big enough, and exciting enough, with enough stuff to say, to make for a decently titillating segment on the TV. There might even be video with flames and wreckage, and there will be plenty of grieving loved ones to parade in front of the cameras.

But there is also a random element to mass media attention. Shark attacks go on at a pretty constant (and low) rate, but every once in a while, there's a media feeding frenzy and every shark attack anywhere in the world gets the O.J. treatment. For a while everybody is afraid to go into the water, then after a while we all forget about it.

And so comes the dreaded swine flu now.

First, here's my scientist's take on it: Influenza is part of the human condition. Hundreds of thousands of Americans get the flu every year, and we've just about all had it more than once. It's a drag, but it's no big deal. You stay home for a few days then you're over it.

Now, the CDC reports variously that something on the order of 40,000 deaths in the U.S. are caused by influenza every year. That's actually debatable, on a couple of grounds. There's hardly ever any confirmation that people actually had influenza: they might have had some other viral illness.

Death statistics actually say "influenza or pneumonia," and there are more than 60,000 deaths listed in that category, and they more or less guesstimate how many were really influenza. But most of those people were old and debilitated, or their immune systems were compromised, so the cause of death listed on the death certificate is misleading.

Tragically, however, around 150 children in the U.S. die each year from complications of influenza. As with the shark attacks, occasionally the media notices one of these deaths, and there's a big freak-out for no good reason. It happens, that's all.

Once we get past perinatal deaths and deaths from congenital conditions, the leading cause of death for children in the U.S. is motor vehicle crashes (7,677 in 2003). The second leading cause? Murder (3,001). So influenza isn't exactly a major scourge of children.

As of now, there is no evidence that this new strain of H1N1 flu is any more dangerous than ordinary flu. Evidently the Mexican authorities thought it might be, but it is not clear why. It has not been reported that there was an unusual number of people showing up in Mexican hospitals with flu until after the government whipped up major publicity about the outbreak, nor is there any evidence as yet that the fatality rate from this strain was unusually high in Mexico. It definitely is not anywhere else.

In Mexico, and all over the world, the amount and nature of flu activity at this moment is completely normal. The number of cases is not unusual, and the number of severe cases or fatalities is not unusual. The only thing that's unusual is that laboratory analysis identifies the strain of flu that some people have as a novel one. That's all.

Now that we have widespread surveillance systems in place, and laboratory tests that can rapidly analyze genetic sequences of viruses, we could make this observation. The fact is, if this had happened 10 years ago, nobody would have noticed it, at least not yet, and the way it's looking to me right now, they probably never would. Comparable events have undoubtedly happened many times over the decades - a new strain of flu emerging near the end of the season - and we just never knew it.

Public-health authorities have a responsibility to be vigilant and to prevent bad stuff from happening if they can. Since novel strains of influenza might, in principle, be unusually dangerous, they look for them and try to a) stop them from spreading if they can, and b) prepare for the worst in case they can't. That's their job.

They have various technical terms and policies they resort to based on their careful planning. For example, they may declare a "public health emergency" in one or another jurisdiction. Nationally, here in the United States, that just means that relevant government agencies assign staff and line up resources to take certain actions in case they are needed, such as shipping stockpiles of drugs to particular locations.

A "pandemic" just means an epidemic that is very widespread. A pandemic can refer to a minor disease. Actually it's technically correct to say that there is a pandemic of one or more strains of influenza every year, but the WHO doesn't bother to use that term unless they consider the pathogen to be particularly problematic.

In this case, they have issued a pandemic alert because they consider it likely that this novel strain will be transmitted from one person to another in many places around the world. That doesn't mean anything unusually bad will necessarily happen. It's just a term of art. They want to be ready.

So the authorities, for the most part, haven't overreacted. They've just done what they are supposed to do. It is possible, but not at all likely, that this virus will cause an unusual amount of trouble.

It is much more likely that it will fade away in a couple of weeks, because the flu season is nearly over, and there is no evidence that there is anything unusual about the way this virus behaves. It might re-emerge in the fall, as a component of ordinary seasonal flu; or it might re-emerge as something more virulent, but by that time we'll have a vaccine in mass production. So everybody needs to chill.

The news media, however, have seriously overreacted, as have a lot of commentators and public figures who probably ought to know better.

There are screaming headlines about this at the top of every front page. The local and national television news programs are filling half of their broadcasts with pointless updates.

I surfed past one local Boston station last night that was doing a roundup of infections in New England: "Three infections have been confirmed in Maine, two in Kennebunk and one in Lewiston. All the victims are recovering." Three people in Maine had the flu and they're all getting better now? This is news? How about "J.B. McPheeters, of Bar Harbor, recently purchased three pairs of underwear?" That would seem to be of approximately equal general interest.

The result is some very real damage. The Egyptian authorities, for example, have ordered the slaughter of all the nation's pigs. This may be an opportunistic move to cultivate favor with religious fanatics, but it's utterly insane as an infection-control measure. You catch the virus from people, not pigs. Joe Biden is telling us all not to ride on the subway or fly. Schools are closing all over the country. John McCain wants to close the border with Mexico. One Harvard dental student was diagnosed, and they have shut down the entire Harvard medical campus. That'll do a lot of good - especially since they've kept Brigham and Women's, Beth Israel and Children's hospitals open, and all of the doctors and residents and medical students are still going there, as far as I know. This means real economic damage at a time when we can ill afford it, and it is utterly pointless.

Why is this happening? Well, it's a novel event, for one thing, although the novelty is largely imaginary. People have a deeply rooted dread of epidemics because of what they have learned about history, and sure, it could happen, and it could be really bad - although at the very worst it will be completely over in less than a year, and it will kill fewer people than tobacco during that time.

The uncertainty, the total lack of control that we each feel, the salability of a news story that could affect any one of us, all these figure into it. It did not help that the U.S. government chose to kick off the frenzy by having the secretary of homeland security (of all people) announce a "public health emergency" while flanked by several other very senior officials. That automatically makes it a big deal.

It also doesn't help that a couple of leading public health bloggers - I'm not going to name names - have been obsessed with the possibility of pandemic killer flu for some years now. They've been all up to the scuppers with H5N1 bird flu, which has so far disappointed, but this came along to save their act.

Don't get me wrong - I don't have a crystal ball, I'm not prophesying that nothing worse will happen. Conceivably it might. But all of this hoopla now isn't going to help. The time to start yelling is when the sky starts falling, and so far it isn't.

Alas, I suspect that more than anything else, there's actually something attractive and exciting about the prospect of global catastrophe.

Remember, a significant number of people actually wanted civilization to collapse on Jan. 1, 2000. There was real disappointment when nothing happened. Life is dull, the world isn't what we want it to be, this is something happening.

The worst part, for me, is that we should be talking about public health a lot more, and in a much more sophisticated way. Public health is about justice, and equity, and security and community. It's all about progressive values. But we aren't having the discussion we need to be having.

Instead we're losing our minds over a phantom.