In this short lecture, Dr Rocha Martin invites the public to take a tour inside the body. We will stop at all the places where microorganisms live within our bodies, aiming to understand how they can protect or harm us. The first stop is our skin, the largest organ we have, which is covered with microorganisms: the skin microbiota. Then we will travel through the mouth and down our gastrointestinal tract to the colon and the gut microbiota, a huge and diverse microbial community that has more microbes than the number human cells in the body. Finally, we will find out how microbes can help crying babies and their parents!