You’re either floored by flu – or you don’t even know you’ve got it

In a 2011 study, only 25 per cent of patients who tested positive for influenza actually had any symptoms.

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There’s an old adage: once you’ve had influenza, you will never refer to a simple cold as “the flu” again. I can testify to that. I contracted flu while a first-year medical student; I’d never felt so ill in my life. I spent days huddled beneath a duvet in a darkened room, wracked by horrendous muscle and joint pains, with a head that felt like it was exploding with every cough.

Worried friends in my hall of residence contacted the university GP. I remember him poking his head round the door, peering at me through the gloom, and muttering, “It’s flu, you’ll get over it”, before disappearing. I’m not sure that would qualify as an adequate assessment in these more meningitis-aware times, but I guess I fitted a pattern he recognised around campus. It was also many years before the introduction of seasonal flu vaccinations – I doubt he wanted to get within six feet of me lest he contract it himself.

What makes flu so much more severe than a cold? Both, after all, are viral infections of the respiratory tract, but most people can soldier on through a cold, whereas flu has a reputation for incapacitating even the fittest, strongest patients. I’ve pondered this question over the years but never got round to answering it. Until now, that is, when my second bout of influenza has given me motivation for research, and afforded me time (if not energy) to conduct it.

Perhaps surprisingly, the symptoms of colds and flu are not caused by the viruses themselves. Rather, they’re a result of the chemical messengers – called cytokines – that our immune systems produce in response to microbial attack. Cytokines released into circulation in sufficient quantity cause fever, crippling joint and muscle pain, profound fatigue and headache, and also impair cognition.

The difference between colds and flu is one of degree. Cytokines are produced in response to tissue damage. Most cold viruses are only equipped to infect cells lining the upper respiratory tract (nose, sinuses) so they provoke relatively limited cytokine release. Flu viruses are able to attack the entire breathing system. They have a fondness for the voice box, windpipe and tubes leading into the lungs, but can damage even the tiny gas exchange spaces deep inside the substance of the lungs. Widespread tissue damage causes much greater cytokine release, so far worse symptoms (it’s why flu can be fatal in susceptible groups).

So far, so convincing, but some recent research suggests this is only part of the story. In 2011, American scientists signed up healthy volunteers to be exposed to the flu virus, then monitored them with frequent blood tests. Half the study group developed symptoms of influenza, half stayed entirely healthy. But all of them showed biochemical evidence of active viral infection and immune system fightback. This surprise finding also helps make sense of the Flu Watch study in the UK a few years ago. Only 25 per cent of patients who tested positive for influenza actually had any symptoms.

My family were distinctly nonplussed by me bringing my work home like this. Out of courtesy, I kept my distance from them. I’m now past the infectious phase, and – touch wood – no one else has become ill: perhaps the semi-quarantine was helpful. But my wife is sceptical, and I suspect she’s right. Given the infectivity of these things, it’s far more likely that they’re simply in the 75 per cent who develop no symptoms.

It’s not yet clear why some get floored by flu and others don’t even notice they’ve got it. Whatever the answer, it seems likely it will lie in how the immune system reacts to the infection. Some people may be constitutionally geared to eradicate flu swiftly without much cytokine release. For most of us, though, the better rested, fed and exercised we are, the brisker our immunity and the more likely we are to stay well. The fact I came down with flu – in spite of having had a jab – may be telling me I need to ensure I get a bit more sleep.

Phil Whitaker’s sixth novel, “You”, is published by Salt

This article appears in the 04 April 2018 issue of the New Statesman, Delusions of empire