John M. Barry brings unsettling news from the frontlines of H1N1 research: this novel influenza virus is very hard to pin down. In spite of international scientific scrutiny, H1N1 continues to baffle and elude, worrying health officials defending against the pandemic, and challenging some ideas about influenza in general. Says Barry, “A lot of things we thought we knew, the virus demonstrates we knew wrong.” Barry examines the current pandemic in both historic and scientific context. Most influenza viruses share certain features: They can jump to other species by way of mutation, or by mixing genetic components with another virus that happens to be infecting the same cell at the same time. Influenza pandemics go “as far back in history as we can look,” with 10 occurring in just the last 300 years. Four of the most recent pandemics appear to have rolled out in waves of varying lethality, infecting at peak times some 30% of the human population.  Before last year, the latest pandemic threat seemed to be H5N1, an avian flu jumping to humans. But, says Barry, “while we were all looking at H5N1, this H1N1 virus snuck up on us and we have no idea yet how serious it will be.” The problem for researchers is that H1N1 simply won’t behave in predictable ways. When ordinary influenza viruses are transmissible between humans, novel molecular markers are present. The current H1N1 doesn’t bear these markers, yet is transmissible. There are conflicting reports on whether this flu is more infectious than the seasonal flu. There’s evidence that some people over 60 are resistant, perhaps because they carry antibodies to previous influenzas. And although H1N1 doesn’t exhibit conventional molecular tags for virulence, it is virulent. Unlike seasonal flu, when H1N1 kills, it targets younger people, and it does so through viral pneumonia, as opposed to complicating bacterial infections. “Depending on how you ask the question, it’s either extraordinarily mild, more mild than s...