The past few years have seen one of the biggest, if weirdest, revolutions in medical science – the discovery that dozens of species of tiny organisms are helping run our bodies. These internal bacterial guests make up the human microbiome and are intimately tied up with health.

They affect not only your digestion but also your immunity, heart health and ability to maintain a healthy weight. They can even influence the likelihood you’ll live past 100 and your risk of developing Alzheimer’s or Parkinson’s.