Before I read this book, I imagined the immune system as a defensive force, like the Germans on the beaches at Normandy on June 6 1944. When you’re young and vital, your immune system is the Germans in the early morning — scanning the horizon for movement, with plenty of ammunition in reserve. But life is a process of attrition; as you get older, you become like the Germans later that afternoon — your machine guns get jammed up, and then you use rifles, and pistols, and eventually bayonets, until the invaders finally destroy you — just like the first 20 minutes of Saving Private Ryan.

That’s what I used to think. Now I’ve read this book, I see things a bit differently. Of course, I was right in part — our bodies do have a border patrol, the mucosal immune system, which is responsible for more than 300 square metres of vulnerable territory. The mucosal immune system protects the soft, wet surfaces where our bodies come into contact with the outside world — mouth, lungs, eyes, guts. Idan Ben-Barak describes it as

a collection of frontline military units engaged in a never-ending low-intensity conflict at an open border, involving a complicated relationship with the civilian population, which may or may not contain hostile elements at any given point.

Here’s the thing: as soon as you begin to scrutinise it, you see that the the conflict between disease and immunity is not like a simple battle, with clearly defined goodies and baddies blasting away at each other. It’s much more like a modern cyber-conflict. It is, says Ben-Barak, about ‘intelligence, counterintelligence… misdirection, disguise, decoy, deceit, logistics and so on’. At the cellular level, a lot of your enemies dress up to look like civilians, and sometimes they manage to brainwash civilians, who then join the enemy forces. Many cellular participants are prepared to commit suicide. Reading this book, I began to visualise the world of the immune system much less as the first 20 minutes of Saving Private Ryan and much more as an episode of Homeland.

The more you look at it, the more complicated it all seems. Take the microbacterium tuberculosis. This germ breaks into the lungs, where it is arrested by a defensive macrophage, which tries to eat it and dispose of it in its lysosome, ‘the floating acid-filled chamber of explosive death’. But the germ manages to trick its way into a different part of the macrophage, where it breeds. Streptococcus bacteria dress themselves up in civilian clothing, by gathering local proteins. Chlamydia will penetrate a cell and then switch off the cell’s alarm system. Neisseria gonorrhoeae goes one better, ‘in effect sending out a falsely reassuring signal that prevents the immune system from springing into action.’

I’m only scratching the surface here. Nasty things sneak into our bodies, having done their homework, and disguise themselves as parts of us, or find good places to hide, while they carry out their destructive tasks. Like terrorists, sometimes these tiny migrants destroy us. But they don’t want to destroy us. What they want is to find a nice place to live and plenty of food. A Ukip spokesman might say they want to bypass our border control and live off our benefit system. But then again, repelling all of them is counterproductive, because lots of bacteria are beneficial — and the debate about which ones, and how many of them we need, is maddeningly complicated.

In fact, there are lots of things we’re only just looking into. For instance, conditions such as ADHD might have something to do with the fact that we live in environments that are too sterile. So maybe we need more germs.

In the end, this is a nice chunk of popular science. Ben-Barak keeps his health advice to a minimum, which some will see as refreshing. Here it is:

Eat well, sleep well, move more, drink in moderation, don’t smoke anything legal, vaccinate, and don’t be too fussy about dirt.