Whether it's an Al-Qaeda-produced smallpox or monkeypox variant with a week-long incubation period, an Ebola or Marburg variant being transmitted accidentally at airports, or a suddenly-intense mutation of avian influenza, the impacts would be dramatic. See Laurie Garrett's magnificent The Coming Plague, winner of the the Pulitzer Prize, for more information, as well as The Impact of Globalization on Infectious Disease Emergence and Control, National Academies Press, 2006. The government, and the public health system, unfortunately would be unlikely to effectively respond with vaccine development, distribution, and direct implementation.

We are hypothesizing a disease with a slow incubation, extreme transmissibility, and a death rate of 10-30%. This is actually low for many of the possible pathogens. This sort of pandemic would likely drive much of the following:

• A dramatic decrease in the use of people-heavy places: airports, malls, workplaces, grocery stores, buses, sports stadiums, you name it -- we'd all be terrified that if we go out of our homes, we'll catch it.
• Serious impacts on infrastructure stem from workers calling in sick, or taking paid personal leave: power systems, transportation systems, commercial sales, even most "knowledge work" offices will be fairly barren, for a good long time.
• Hospital systems could break down: not enough beds, not enough ability to quarantine, lots of people with damaged immune systems, and worse. Developing and distributing vaccines or medicines may be troubled, as bottlenecks and a lack of living nurses and doctors may create "viral riots" in some areas.
• Refugees are likely, but entire communities will (like what happened in the Black Plague, or the Great Influenza Epidemic) quarantine themselves, and drive off the desperate, potentially-infected refugees.
• Economic collapse could quickly follow: international travel/ shipping, and even interstate travel/shipping of food and medicine, will be seriously affected.
• National panics (imagine Fox or CNN's breathless treatment of a new plague) could quickly create hoarding, violence, estrangement, and other desperate, counterproductive measures.
• Massive deaths create new horrors and health issues, as rotting bodies are left where they fall.
• Internet use, where network systems are able to be maintained by telecommuting or clean-room-ensconced systems administrators (see Cory Doctorow's Nebula-winning When Sysadmins Ruled the Earth), becomes vital to survival. However, vast areas of the Web go dark, as key figures succumb.
• Cities quickly become uninhabitable, at least for a few months, though these population centers will likely get any vaccines or medicines early.
• Order and rule of law collapses, worldwide.

There are plenty of sub-scenarios where contagion doesn't fully apply; where antigens, antivirals, and/or inoculations hold sway. We hope that happens. But we think it more likely that infection, uncertainty, and accidental transmission will lead to a general population who are fearful, uncertain, and reactive. In that case, society will shut down, at least for a few weeks, which (in this fragile just-in-time economy) is enough to cause catastrophe.