Introduction: Why This Primer Exists
You are about to read a book about the trillions of microorganisms that live inside and upon your body — how they got there, what they do, and why they matter for your health, your mood, and perhaps even who you are. That book, The Inhabited Body, assumes no specialist knowledge. Every technical term is explained on first use. Every complex idea is introduced with an analogy before the details arrive.
But here is the reality: some ideas in biology need more than a sentence of explanation. They need a running start.
If you already know what DNA is, how a virus differs from a bacterium, and what it means when scientists talk about "sequencing a genome," you may not need this primer at all. Feel free to jump straight to Chapter 1 of the main book. You can always come back here if you hit a term or concept that catches you off guard.
If, on the other hand, the last time you thought about biology was in secondary school — or if terms like mRNA, archaea, and metagenome feel unfamiliar — this guide was written for you.
What This Primer Is
This is a short, self-contained introduction to the biology you need in order to follow the arguments in The Inhabited Body. It covers six topics:
- The living world — the major categories of life on Earth, from bacteria to fungi to the strange entities (viruses, prions) that blur the boundaries of "alive."
- The cell — what cells are, how they work, and why the cells of bacteria, archaea, and humans are built on fundamentally different plans.
- The code of life — DNA, RNA, proteins, and the molecular machinery that turns genetic instructions into functioning organisms.
- Fungi — a kingdom of life that is often overlooked but increasingly important to the microbiome story.
- Viruses and their relatives — including the bacteriophages that shape microbial communities in ways we are only beginning to understand.
- How we study what we can't see — the tools and techniques that underpin every claim in the main book, from microscopy to metagenomics.
Each chapter is written to stand alone. You can read them in order — they are arranged in a "zoom in" sequence, from the broadest categories of life down to the molecular details — or you can dip into whichever chapter addresses a gap you have noticed. Cross-references to the main book are provided at the end of each chapter, so you can see exactly where each concept becomes relevant.
What This Primer Is Not
This is not a textbook. It does not attempt to be comprehensive. There are entire university courses on each of the topics covered here, and this primer compresses them ruthlessly. The goal is not mastery but orientation — enough understanding that, when the main book describes a bacterium exchanging genes with its neighbour, or a phage altering the immune response, or a metagenomic study revealing an unexpected community of organisms, you know what is being talked about and why it matters.
Where simplification risks distorting the science, we say so. Where a topic is more complicated than the explanation suggests, we flag it. Science is full of caveats, exceptions, and ongoing debates, and pretending otherwise would be doing you a disservice.
A Note on Language
Biology has a terminology problem. Many of its key words — gene, species, alive — mean slightly different things depending on context, and even experts disagree about precise definitions. In this primer, we use each term in its most common sense and explain it in plain language. Technical terms are bolded on first use and defined in the glossary at the back.
Where a word has a Greek or Latin root that helps make sense of it, we mention it — not because etymology is essential, but because knowing that prokaryote means "before the kernel" and eukaryote means "true kernel" makes the distinction between these two types of cell easier to remember. You will find, over time, that the jargon stops being jargon and starts being a useful shorthand.
How to Use This Guide
If you are reading The Inhabited Body from the beginning, you may want to read this primer first, or at least skim the chapter summaries in the table of contents to see which topics you are already comfortable with.
If you are already reading the main book and have hit a wall, the cross-references at the end of each primer chapter will tell you which primer chapter covers the concept you are struggling with. The glossary can also serve as a quick-reference tool.
If you are simply curious about microbiology, this primer works perfectly well on its own. It covers the fundamentals of how life is organised, how cells work, and how scientists study the invisible world — topics that are interesting in their own right, whether or not you go on to read about the human microbiome.
However you use it, we hope it makes the science in The Inhabited Body not just accessible but genuinely enjoyable. The microbiome is one of the most exciting frontiers in modern biology. Understanding it starts here.
Let us begin with the broadest question of all: what kinds of life exist on Earth?