Begin your study of the oldest and most widespread forms of life on Earth: microbes. Dr. Melissa Booth outlines the course and introduces Lina, the fictional character whose life will be explored from the perspective of the microbes that live in, on, and around her, from birth to adulthood. Learn how the microbes that colonize an infant during its earliest weeks influence long-term health.

Continue your tour of Lina’s microbiome by surveying the five major types of microbes: bacteria, archaea, fungi, protists, and viruses. Examine what they do and where they live in the human body. The statistics are astonishing, since just over half the cells in your body are microbes, comprising some 39 trillion microbial cells versus 30 trillion human cells. Plus, we harbor 380 trillion viruses!

Any part of the human body that has contact with the outside world is a potential homestead site for microbes. But why do our bodies tolerate their presence? Here, you’ll answer the question: What are these microbes doing for you? One of their important jobs is to disarm potentially harmful microorganisms. Discover how they also aid in digesting complex molecules such as gluten.

Follow the steps that transform cacao beans into chocolate as an example of fermentation—an intricate process mediated by different microbes. The ability of certain microorganisms to turn nutrients into energy in the absence of oxygen is what makes fermentation possible. Also, look at wine, cheese, and yogurt production, and consider the health benefits of fermented foods as well as the risks.

Explore the diverse environments of the human gut from a microbe’s point of view: the deadly acidic cauldron of the stomach, the constant antimicrobial rain in the small intestine, and the paradise of nutrients in the colon. Learn how good health depends on microbial fermentations churning away in our bellies. Every gut has its own unique mix of microbe species, but their functions are the same.

Microbes are known to make creatures do strange things, such as zombie ants that climb to a specific height to die and spread the infecting agent. Rabies is another example since it alters animal behavior to promote spread of the virus. Dr. Booth investigates whether microbes within us may hijack our minds for their own benefit. Discover that one line of defense is the blood-brain barrier.