Swine Flu has been all over the news the last couple of days. Lots of people are using the word pandemic. Is this the big one? Personally, I doubt it. Governments got very scared about SARS and Bird Flu in the last few years, so I think the appropriate agencies are relatively well prepared for this.
Just a word of advice - when it comes to science and medicine, don't listen to your friends. They don't know anything (unless they're, you know, doctors and scientists in appropriate fields). Find authoritative sources. The Center for Disease Control, scientific websites like New Scientist and Nature and professional health-focused sites like WebMD are a better source of information than that guy you talk to on the bus. Even the usually terrible local evening news is slightly better. Misinformation is very dangerous during an emergency.
So, after all that let's get all worked up. This is all hypothetical. What would an apocalyptic swine flu pandemic look like?
If we look at a real pandemic like the 1918 Spanish flu, which killed about 100 million people, it's easy to see that a disease on that scale isn't really apocalyptic in nature. The Spanish flu killed between 2.5-5% of the human population. To be apocalyptic it would have to go way beyond that and kill maybe 25% or more, like the Black Death did in 14th century Europe.
For the sake of argument, let's assume that it has high lethality, is resistant to treatment, spreads easily and ultimately kills about 25% of the population, spread around evenly. Since it was caught after quite a few deaths in Mexico, and there are known cases in the US, it's a safe bet it has spread to other locations via major travel hubs and large cities - LA, NYC, London, Paris, Hong Kong, Beijing, etc... If things go well any infected people will be quickly identified and isolated and the spread of the disease halted in that area. If they don't go well once these major travel hubs are thoroughly infected they will be next to impossible to lock down. The disease will spread out to smaller cities and towns as panicked people head away from the bigger cities.
Once the disease really starts to infect large numbers of people and the authorities try to restrict travel we can expect to see problems with resources. I think this is especially true of places like the US and Europe, where there are not necessarily a lot of local resources of particular types because it's cheaper to centralize and ship long distances. For example, car manufacturers might have a hard time getting steel because their trains are restricted or there are too many sick engineers. Certain goods will become very hard to find, for instance tropical fruits in the upper midwest US. Obviously the authorities will try to keep medicine and necessities in good supply, and if nothing else happens to make it worse, the effects of the pandemic will probably end somewhere around there. Lots of people dead, but little else truly impacted. Critical infrastructure and government will likely remain in place and functional.
As long as the percentage of deaths doesn't go too high, there will be a shifting in people's focus. People involved with luxury items and non-essentials will shift to fill the gaps in more essential services. We might see a drop in dog groomers, florists and baristi and an increase in local farming and manufacturing jobs. Some other interesting fallout would be a wide-scale housing market upset, as so many vacant homes come on the market. You won't be able to sell your house because it's so easy to get a cheap vacant one. People who are renting currently will be able to cheaply move into their own houses and the rental market will be in big trouble.
For a few years after the pandemic I expect there will be a kind of resistance to large social gatherings, or even the use of public transportation. The car market may boom despite recent environmental concerns as people shy away from the close quarters of the bus and train. Telecommuting will become huge, first because it will let people work while ill without any risk to coworkers, and second because it will be necessary to recruit people from farther away if there are shortages in skilled workers locally.
The developing world is going to be hit much, much harder than the US and Europe. Medical supplies are already limited, and areas that are already experiencing war, famine or the AIDS epidemic may be all but wiped out. Places that loose too much of their population will see their basic infrastructure collapse, and the rest of the world will be ill-equiped to assist.
All in all, it would be a pretty scary and nasty event, but compared to the other big potential apocalypses (nuclear war, environmental collapse and zombies) this one is pretty mild. The human race will certainly survive and the world won't be a wretched hellscape for generations. There may even be some upshots. After facing a brush with apocalypse, people might start to think in longer terms than they did before and plan more effectively for this kind of thing. A drastic reduction in population will also cause a drastic (temporary) reduction in environmental impact which could slow the approach of some of the environmental dangers looming on the horizon.
All this is just my opinion, obviously. What do you think could happen during or after an apocalyptic plague? Let me know in the comments.