Yet not all microbes are bad. Yes, there are bacteria that cause unpleasant or even deadly diseases, but lots of them are extremely useful and beneficial to our health. They make vitamins in our gut, coat our skin to protect us from harmful microbes, and help us digest food. Outside of our bodies, they decompose organic waste, make half the world's oxygen and fix nitrogen levels in the air – helping make the Earth the life-supporting planet it is. Today, many scientists argue that people have become “too clean” for their own good. In 1989, the British epidemiologist David Strachan was the first to suggest that the exposure to infections during childhood would provide a good defence against allergies in later life. It’s an idea known as ‘Hygiene Hypothesis’.