A new headline-grabbing report from the White House claims that swine flu could plausibly infect up to 50% of Americans, causing flu symptoms among some 60 to 120 million of them, and leading to as many as 1.8 million hospitalizations and 30,000 -90,000 deaths.

Where, exactly, do numbers like these come from? The new report was put together by the President’s Council of Advisors on Science and Technology. It turns out the predictions are based on just a couple key facts: