Petya is an ordinary boy. Just like other children, he gets sick sometimes. One day Petya ended up in a hospital and there he became friends with Dr. Tolya, who talked about the fascinating world inside the body and showed this world in drawings. And Petya wanted to share his knowledge with all the guys. On the pages of the book, the boy tells how children are structured, describes all the systems of the body using the example of his body and shows the process of development of the disease: from the moment of contracting the infection to recovery.