The human body is home to some 30 billion microorganisms (bacteria, fungi, viruses, algae). The composition of this microbiome has a direct impact on us: the more diverse it is, the better our health. However, urbanisation, hygiene regulations and changing diets and lifestyles are leading to a loss of microbial biodiversity. So how can we bring back the microorganisms that have disappeared into modern life? And how important is it to us really?