Influenza, commonly known as the flu, is one of the most common viral diseases on the planet.  There is a flu virus for almost every mammalian and avian species on the planets, and influenza kills approximately 36,000 people each year in the United States and hundreds of thousands worldwide.  In addition to the human cost, the flu is responsible for billions of dollars in economic damage due to lost time at work and health care costs.  Occasionally, flu will reach pandemic levels (a pandemic is an epidemic on a global or supra-regional scale) and can result in millions of deaths, as was the case with the 1918 Spanish Influenza (50-100 million deaths worldwide) and, more recently, the 1968 Hong Kong Influenza (~700,000 deaths worldwide). 

Given that the flu is such a nasty virus under "normal" conditions, i.e., without a species-jump event, (it is likely that a species jump from birds to humans caused the 1918 pandemic), when something like swine flu starts to show up, it is a cause for concern.  However, it is usually best to temper this concern with facts before it turns into a needless panic (SARS, anyone?).  So what then, is the flu? To answer this question we'll have to get a bit technical, but simple explanations never work for complex problems anyway, so stick with me here and we'll see what the flu is, how it works, and why you should be concerned.

Like all viruses, the flu is pretty simple.  At it's simplest level, it is a vehicle for sneaking its genetic information (the flu is an RNA virus) into host cells.  The flu only has 8 genes and each gene is carried on its own segment of RNA.  These segments are contained within a protein capsule comprised of the proteins hemaglutinnin and neraminidase (this is why flu strains are referred to as H1N1, H5N1, H3N2, etc.), with the hemagglutinin proteins sticking out like spikes (see figure below).

Photo Courtesy of: New York State Department of Health (here)

These spikes play a key role in infection.  Their main role is to bind to sialic acid sugars found on the host cell's outer membrane, which will then lead to the virus particle being taken up by the host cell.  This process is very complicated, but since different species have different sialic acid sugars, and since recognition is very specific between hemagglutinin proteins and sialic acid sugars, not every type of flu virus will be able to infect every mammilian/avian species.  For example, the hemagglutinin H5 (the type found in "bird flu") can currently only recognize avian sialic acid sugars, so unless it mutates and gains the ability to recognize human sialic acid sugars it will not pose a significant threat.  Swine flu, on the other hand, is an H1N1 type virus - the same type that cause the Spanish Flu pandemic, and now one of the more common flu strains - which means that it is able to infect humans fairly easily.  It's best to keep in mind, however, that this H1N1 type has been around for the past century, so our immune systems are fairly well adapted to it.

Once the flu gets inside your cells, it starts to replicate.  To do this, it hijacks your cellular machinery, using your proteins to replicate its genome.  Once enough new virus particles have been made (about 100 million per cell), the cell will burst and these new virus particles will find new cells to infect.  Normally, your immune system will recognize foreign proteins fairly quickly and produce antibodies which can destroy them.  Since the hemagglutinin protein is necessary for cellular uptake, this protein is often targeted by the immune system, so it would seem that stopping the infection would be fairly trivial - just send out antibodies to destroy the hemagglutinin.  However, influenza has two main factors to combat this immue reponse - a high mutation rate and scale.  Like all other RNA viruses (HIV is another), influenza mutates rapidly.  When this mutation rate is combined with 100 million new particles per replication cycle, every possible genome is reached quickly, which leads to some virus particles having hemagglutinin proteins that are just different enough to escape antibody detection but still specific enough to invade cells. 

It's precisely these two factors that make swine flu so problematic - our immune systems have never seen this specific variant before, so it will be much harder for our immune systems to mount an effective response, which could lead to a cytokine storm (discussed in much greater detail at the link).  The worst possible scenario would then be a return to the death curve seen in the Spanish Flu pandemic (see figure below), where young, healthy patients are also severely affected (the "W" shape in the solid line).  As the figure shows, influenza is normally (dashed line) only fatal to the very young and very old because these groups tend to have a weaker immune system and are unable to fight off the infection.  An alternate explanation for the W-shaped curve is that young, healthy people have not experienced the specific virus type before, so their immune system has no memory of it - making them susceptible to infection as well.

Graph courtesy of US Government files.

Now that we've seen why the flu does what it does and why swine flu could  be a very nasty bug indeed, it's time to discuss what other factors we need to know before we let the panic set in.  The first and most important factor is the mortality rate.  Currently, it is estimated that 149 people in Mexico have died from swine flu, but it is not known how many were infected.  If 150,000 were infected, then only 0.1% of cases are fatal (not a big deal), but if 1,500 were infected, then a staggering 10% of cases are fatal (a very big deal - mortality from the Spanish Flu pandemic is estimated to be between 2.5% - 5.0%).  The lower the mortality rate, the better.  Just looking at the mortality is also not enough - in order to be truly dangerous, the disease must be easily transmitted by person to person contact.  If people can't spread the disease amongst themselves, then only those in close contact with infected pigs are at risk, but if sneezing can spread the disease, then you can catch it from the sick person on the bus.  Sadly, the WHO has recently determined that swine flu is indeed highly transmissible - so it is smart to take precautions (see below).  

Transmission rates and mortality are also not enough to accurately assess the danger - the conditions surrounding fatalities must also be taken into account.  Did those that succumbed have access to hospital care?  Did they even go to a hospital?  Where they already immunocompromised due to AIDS or other pre-existing conditions? At this juncture there is simply not enough information to make accurate statements, so caution (but not panic) should be urged.  Check the updates posted by the CDC (see the swine flu tracker at the top of this sidebar too) and the WHO as more information comes in - these organizations will be your absolute best source for reliable and up-to-date information.

The final concern is distribution.  If swine flu remains localized to North America (which is looking less and less likely every day, as cases have been reported in Spain and New Zealand) then a global pandemic is less likely.  However, since it has already been confirmed in Europe and Oceania it is now highly possible that it will become a global pandemic.  This development is obviously a Bad Thing, as swine flu would then hit various population centers, mutate, and then make another trip around the world, a pattern seen in both the 1918 and 1968 pandemics.  CNN has posted a mildly alarmist article about this, so I'll summarize briefly.  Worst case scenario: 18-24 month duration, 2 billion infected, up to 7 million people dead.  Being a good scientist, I'm going to have to wait a bit to make any realistic predictions, but I certainly hope that we will not see the worst case scenario.  This possible crisis promises to bring a few very interesting weeks, and given the possible outcomes it would be very wise indeed to keep yourself informed.  See below for things you can do and links to keep updated/learn more.

If you'd like to learn more about the flu in general, and the 1918 pandemic in particular, I highly recommend going to iTunes and checking out the two featured lectures they have under the iTunesU tab - both are free, informative, and well-made.

What Can You Do?

I am not a physician, so please, remember to be smart and not let advice on the internet supersede or worse, replace, advice from a qualified medical professional.

1. Wash your hands frequently.
2. If you're sick, stay home.
3. If you are very sick and have flu-like symptoms, see your doctor.  Otherwise, stay home.
4. Avoid contact with other sick people.
5. Avoid touching your mouth, nose, and eyes after being in public.
6. If you do go to the Emergency Department, please wear a mask - if you get all of the other people there sick you will spread the virus much further and possible contaminate the whole hospital!  This is not a joke; hospital-acquired infections are a leading cause of death.   While wearing a surgical mask won't protect you from infection, it can prevent you from spreading the disease.
7. Don't panic - this outbreak is still in the early stages and it still too early to make any accurate predictions.  Panic can only make the situation worse.  If there is a significant risk, governments are likely to close schools and other public places - the best measure that they can take.

Helpful Links

What to do if you're sick with the flu
Swine Flu Updates
CDC
WHO
Flu Redux
Practical Tips for Combating Swine Flu in Your Home