Here in the northern hemisphere, flu season generally starts in the fall and rapidly falls off at the end of winter. This past flu season, we had a "two-fer", with an initial dip in cases, followed by a spike in new cases attributable to the novel H1N1 ("swine") flu that emerged late last year.
The bimodal flu season of 2008-09
This is my first pandemic, and it's been fascinating. The sudden drop in cases at the beginning of summer was matched with a sudden increase in cases in summer camps and military installations. The high attack rate is particularly dramatic, although the virulence is not. Still, even with such a large number of people infected last season, most people have not yet been exposed, which leaves us open for a real whopper of a winter.
Influenza morbidity and mortality are tough to track because we don't test and report every case. The traditional method is using a rise in baseline mortality from pneumonia and influenza. Many who die from influenza actually die from pneumonia and the two diseases can look similar clinically. Also, pneumonia is relatively non-seasonal, so a spike in deaths from pneumonia+influenza that tracks with an increase in positive flu tests is a good indication of "excess" deaths due to flu. This number, which is usually around 35K to 40K per year, is not a measure of all flu deaths, but excess deaths above the usual baseline so actual flu deaths are probably greater.
A certain small percentage of flu patients die in a given year, so the more cases, the more deaths (assuming no changes in virulence). Now imagine that the curve of flu cases shifts upward significantly; even without a change in virulence, there will be an increase in flu-related deaths, so even a "mild" pandemic can stress our health care system. There are ways to prevent flu deaths---vaccination, of course, but also vaccination against pneumococcus which can help prevent deaths due to secondary pneumonia.
It's still not clear what will happen this year, but it's a reasonable bet that we're in for a bad flu season. In addition to the usual seasonal flu, the swine flu is likely to sicken a lot of folks. Here on the front line, we see this as a waiting room full of miserable people, coughing, slumped in chairs. In the hospital, ICU beds start to fill up with flu and pneumonia patients. The CDC tracks this data rather effectively, but a new tool is out there (of uncertain significance). Google has used search data to track the flu, and although it's not clear how good this data is, preliminary evaluation is promising. For the fun of it this year, I'm going to watch both google and the CDC, along with my own clinic and see what shakes out.
But in order to be able to carry out my day-to-day activities, I'm getting both my seasonal flu shot and my swine flu shot the minute they're available (and giving them to my family). I've also made sure my pneumococcal vaccine is up to date, and I've become even more compulsive about hygiene. Not only am I washing or using alcohol-based hand sanitizer like crazy, but I don't touch public doorknobs and other sources of fomites. This may seem a little compulsive (something I'm not often accused of) but it's cheap, easy, and probably effective.
So hang on, it may be quite a ride.