Swine flu is sweeping—if not the nation, then at least the nation's media. But what is special about this virus and what, if anything, should ordinary citizens do about it?

The new flu, which has elements of pig, bird and human flu viruses in it, has been circulating for at least a month in Mexico. In the past week, it has popped up north of the border (with 45 cases confirmed in New York City, 10 in California, six in Texas, two in Kansas and one in Ohio), according to the U.S. Centers for Disease Control and Prevention (CDC), as well as in at least five other countries, The New York Times reported yesterday.

As the number of deaths likely caused by the virus in Mexico reaches beyond 150, researchers and public health officials are investigating intensively and anxious to see what the virus does next.

The respiratory virus, influenza A H1N1, is common among pigs (humans actually first passed it to them after the 1918 influenza pandemic), and the porcine version has been known to infect humans before. But rarely does it make its way from person to person, the way this strain is behaving. The fact that this virus has a sustained ability to pass between people has led the World Health Organization (WHO) to elevate its pandemic alert level to 4 (out of 6, the latter being a full-blown pandemic).

One has only to look back a few years to the 2003 scare over SARS (severe acute respiratory syndrome), which ended up causing about 774 deaths worldwide, for a recent measure of the potential for global pandemic panic. But how does this flu measure up to SARS and to other threatening influenza strains like the H5N1 avian flu virus?

To take the pulse of pig flu, we spoke with Chris Olsen, a professor of public health and head of the Olsen Laboratory, which studies influenza A viruses, at the University of Wisconsin–Madison School of Veterinary Medicine.

[An edited transcript of the interview follows.]

We know that this is an H1N1 strain, so what can that tell us about the virus?
There is not anything inherent about the behavior of the H1N1 virus. What it is important to understand is the H1 and H3 are the only subtypes that have ever circulated widely in the swine populations. Certainly there would be more concern if this were a subtype that had never been seen before…. This is not the same thing as H5 [which is] bird flu.

Do we know what's happening at the cellular level? Is anyone testing it yet?
We don't know that yet. Once we get through the public health crisis phase of things, that's going to be one of the important research questions: to look at differences in infectivity in one species and another. This virus might become quite an important tool in teasing apart the phases of the infection cycle. Right now, the key is to get through the initial public health phase. That comes first in terms of protecting human health and human lives, then we can address the research side.

How does this compare with other flu outbreaks?
There are a relatively limited number of documented cases of zoonotic [human-communicable] swine influenza in the literature. Usually they're limited events where a person was in contact with a sick pig…. This is by far the most extensive person-to-person spread that we've seen from a swine-origin virus.

Why don't people get the flu from pigs more often?
That's a question a number of us around the world have been working on for many years: trying to sort out what controls that species barrier between people and pigs—in both directions.

There's evidence to suggest that people in contact with pigs actually get exposed and get subtle infections more often than the literature would suggest. But either it doesn't produce an illness that's any different than routine influenza or it doesn't produce an illness at all. Likely, we have to point to the specific genetic makeup of this virus [to know why it is behaving differently now]. The $64-million question is: What is that difference?

The silver lining from something like this is it could give us a very powerful tool to help us understand this [outbreak] and help control and prevent a similar one in the future.

Why are avian and swine flus the only ones we hear about as global concerns?
What constitutes a pandemic virus is when a virus enters the human population—a virus we've never seen before in human beings—while everyone is simultaneously susceptible. Where do these new viruses come from? They initially come from avian species. Only H1, H2 and H3 have ever circulated widely in the population. Any of the other 13 [hemagglutinin, or resistance-response, subtypes] has the potential to spark a pandemic.

Instead of going directly from birds to people, it goes into pigs as an intermediary. There, it adapts to a mammalian species, before it is passed to humans.

The flu seems to be affecting mostly young people so far. What does that tell us about the virus?

Older people are more likely [than younger people] to have experienced H1N1 viruses in the past [which means they have built up stronger immunity]. That may be one reason. What, of course, is troublesome about that pattern is that it is the same age trend you saw during the 1918-19 pandemic.

How soon can we expect to see a vaccine?

The CDC already announced that they've begun making the seed strain for vaccine, but that's not what takes the time. What is time-consuming is to actually commercialize that, so that it can be used to make hundreds of millions of doses. Could that be ready in time for fall flu season? Quite possibly.

Has this strain ever been used in previously in flu vaccines?
No. This is a particular genetic combination that has never been seen before in people or in pigs. It's really important to emphasize that although this likely developed in pigs, it is in humans now.

When you think about prevention at this point, it needs to be much more focused on decreased contact with sick people.

How much can Tamiflu and other antivirals help?
The good news is that at this point the virus is still susceptible to neuraminidase inhibitors like Tamiflu and Relenza. Is it possible the virus will develop a resistance to these drugs? It's possible.

What can people who don't have the flu do to avoid it?
The common sense things are avoid contact with people who are currently sick; wash your hands or use alcohol-based disinfectants frequently; avoid contact in particular with people who may have just traveled to parts of the world where we know the virus is circulating; cough or sneeze into your sleeve, not your hand.

This is a time for people to think and use good common sense for personal hygiene and travel. It's not a good time to panic. Panic is not a useful response right now…. It's also important to stress that influenza is a respiratory virus…. There is no risk from handling or eating food—it's not a food-borne disease.

For more, see our In-Depth Report on swine flu.