An ever-changing menace Cavallini James/BSIP/Science Photo Library
Why does flu keep coming back?
Flu is unique among human diseases. It circulates constantly in cool, dry areas of east Asia, conditions the virus prefers, but when temperatures drop during the northern and southern winters, it breaks out and begins a tour of the relevant hemisphere. Because it spreads from person to person efficiently in exhaled droplets, and can be picked up from contaminated surfaces, nearly everyone is exposed.
Winter flu: All the essential facts you need to know
And unlike, say, measles, having flu once doesn’t make you immune to catching it. The virus is uniquely talented at dodging our immune systems. The big haemagglutinin protein on its surface gets most of your immune system’s attention, and this protein constantly mutates at seven hotspots. Every few years it racks up such a number of mutations that many antibodies you made to your last infection don’t recognise the virus, and you get sick again.
You still have some immunity to kinds of flu that are only a little different from viruses you have seen before, which is why much winter flu isn’t as severe as flu can be. The strains best able to evade this kind of prior immunity dominate the annual epidemic in each hemisphere, so we only need one vaccine per season – but a new one each year.
Why are people worried this year?
A record number of flu strains are currently circulating, two in the influenza B group and two influenza A strains, H1N1 and H3N2. H3N2 is the real problem. Our strongest immunity…
