Stool, far from being the remains of our food, is mostly bacteria, some dead, some alive. Around 75 per cent of the wet weight of faeces is bacteria; plant fibre makes up about 17 per cent.

In 2010, a huge study was conducted by a team of hundreds of scientists who hunted through the genes of a quarter of a million people in the hope of finding some that were associated with weight. Astonishingly, they discovered just 32 genes in our 21,000-strong genome that seemed to play a role in weight gain. The average difference in weight

Acquiring the genes to do this is quick and easy for microbes, as their generation times, and therefore opportunities for mutations and evolution, are often less than a day.

A story in which our lives are intertwined with those of our hitchhikers, where our microbes run our bodies, and becoming a healthy human is impossible without them.

Together, the microbes living on the human body contain 4.4 million genes – this is the microbiome: the collective genomes of the microbiota. These genes collaborate in running our bodies alongside our 21,000 human genes. By that count, you are just half a per cent human.

After the small intestine has digested and absorbed as much as it can from what we’ve eaten, the leftovers move into the large intestine, where most of our microbes live. Here, they function like factory workers, each breaking down its own preferred molecules and absorbing what it can. The rest is left in a simple enough form for us to absorb

It’s easy to think of the skin as the barrier between us and the outside world, but for every square centimetre of skin, you have two square metres of gut. Though it’s on the ‘inside’, the gut has just a single layer of cells between what’s essentially the outside world, and the blood.

disruptions to the body’s microbes were behind gastrointestinal disorders, allergies, autoimmune diseases, and even obesity. And it wasn’t just physical health that could be affected, but mental health as well, from anxiety and depression to obsessive–compulsive disorder (OCD) and autism. Many of the illnesses we accept as part of life were not, it

Each of us contains communities of microbes as unique as our fingerprints.