There is only one certainty about flu – there are no certainties.
No virus in human history has sprung more surprises with deadlier effect. Three pandemics in the past century caused millions of deaths around the globe. Constant vigilance is our only defence.
The fear among flu experts is that the novel H1N1 influenza virus that caused the 2009 pandemic, the first of the 21st century, could mutate. We got off lightly last year when swine flu turned out to be a pussy cat not a tiger.
Plans to cope with up to 65,000 deaths in the UK laid in May 2009 were later scaled back to 1,000 deaths, as it emerged people over 55 had some immunity to the virus, owing to previous exposure to a similar strain decades earlier. In the end, there were 494 deaths in the year to April 2010.
A mutation that increased its severity or its virulence among the elderly could cause the death rate to soar. No significant mutation has been observed so far and public health experts are cautiously optimistic that there will be no widespread epidemic this year.
But pregnant women, younger adults, and those with chronic conditions, such as asthma, remain vulnerable to the virus.
Last year's pandemic was the first for 40 years and emerged from Mexico after virus strains in pigs and humans combined to create a new H1N1 influenza virus.
H1N1 swine flu has now become the dominant seasonal virus, driving out the previous seasonal H3N2 virus, and is likely to remain so, recurring each year, for years to come. The expectation now is that the virus will change slightly each year in a process known as "antigenic drift" to infect a new generation. In response, flu experts will gather annually to agree a modified vaccine to protect against the new strain – and any others that may be circulating – until the next major mutation occurs, an antigenic "shift", triggering a new pandemic. This year's vaccine in the UK protects against three strains, including the H1N1 swine flu strain.
But with flu, all bets are off. Predicting the future is a dangerous game. The feature that sets the virus apart is its capacity to surprise. The challenge for global public health is to avoid being caught unawares.