Flu strikes every winter, but this year something seems to be different. A seasonal flu virus suddenly mutated in the summer. It appears to evade some of our immunity, has kick-started a flu season more than a month early, and is a type of flu that history suggests is more severe. The NHS has now issued a "flu jab SOS" as fears grow that this will add up to a brutal winter. There is a lot of nuance and uncertainty, but leading flu experts have told me they would not be shocked if this was the worst flu season for a decade.