Over at Effect Measure some good info on the current Swine Flu situation. I would recommend the whole article.
...If there is normally so much respiratory disease around, why is this an outbreak or even an epidemic? It's a more difficult question than it appears, and it relates to "what did you expect?" An epidemic is an increase in the number of new cases beyond what you would expect. Four or five cases of human rabies in an area in the US would be an outbreak or even an epidemic. Hundreds of colds or even serious pneumonias in an urban area is normal. It's not an epidemic. What makes the swine flu an outbreak is that it is an infection with a virus we haven't seen before and which we believe may be new. Hence these cases are not what we expect and it is an outbreak. If it turned out that there had been the same number of cases in Mexico but from many different known viruses we might look for another explanation, for example, a change in insurance that changed care seeking behavior so cases were counted that weren't counted before.
Another thing that most people and probably most clinicians expect is that we know a lot about influenza. Perhaps because of the increased scientific interest since bird flu (an increased interest which will pay off handsomely in this outbreak, by the way) we do know quite a bit, but we also now know many of the things we thought we knew about flu, like the main ways it is transmitted from person to person, we don't really know. For example, how likely is it that you can get flu by touching a door knob or arm rest that someone with the flu just touched? Or that you can get the flu by sitting in the same emergency department waiting room (but not next to) other flu cases? These are open questions (see some of our many posts on this here, here, here, here). Why is flu seasonal? We don't know. We'll try to get to some more of these questions in the days ahead, since we have many new readers, but one big thing to know was emphasized by Acting CDC Director Richard Besser at the White House briefing yesterday: the influenza virus is highly unpredictable and our certain knowledge of it very scant. If you've seen one flu pandemic, you've seen one flu pandemic.
If this outbreak becomes a sustained worldwide one -- the definition of a pandemic -- you should not expect it to be the same as any other pandemic. It might be like 1918, 1957, 1968 or just a bad flu season. Or not. (more)