Have you ever wondered who else lives inside you? Dr. Silvana Tapia reveals the mystery of the trillions of microorganisms that accompany us from birth to death. Far from being invaders, these tiny beings are an essential part of who we are. With clear and precise prose, the renowned microbiologist takes us on an intimate journey through the microscopic ecosystems that thrive in our intestines, skin, mouth, and reproductive system. She also unravels amazing questions: How do these microorganisms shape our development from the womb? What role do they play in diseases such as cancer, diabetes, and neurological disorders? Can our microbiota influence our mood and mental health? How do these microorganisms communicate with our immune system? Tapia also gives us tools for our daily lives. With useful tips and surprising facts about our habits—covering nutrition, exercise, and responsible use of medications—he teaches us how to cultivate a healthy microbiota through small changes in our routine. After exploring this fascinating microscopic world, you will be left with a transformative certainty: these invisible beings are not casual passengers, but essential collaborators in the journey of life. Taking care of them is undoubtedly the most direct way to take care of yourself.