A human body is a habitat for millions of microorganisms. Many of the cells in our body are not human but belong to tiny creatures that live in and on us. Find out what they are all up to, and whether they are useful for our health or not. What we can easily see all around us is only a fraction of what is really there. The microscopic world in and on animals, plants, the ground we walk on, and even our own body is full of tiny things that are scary, mysterious, and amazingly strange.