Our body is the playground for around 100 trillion microbes, hiding in our mouth, nose, guts, skin and genitals.
In this week's Discovery Geoff Watts visits the Human Microbiome Project, where they're sequencing the genomes of human microbes.
Many are 'good' bacteria, helping us digest food, produce natural moisturisers and synthesise vitamins.
But new research has suggested that 'bad' bacteria may be involved in a diverse range of diseases including obesity, heart disease, cancer, Alzheimer's, arthritis and autism.
The hope is that by understanding more about our microbes we can develop new ways to treat, and even prevent, these illnesses.