It was supposed to be simple.

Back in antiquity, disease came from the gods, or perhaps from God, or — if you were a rational, hard-headed, modern, clinically oriented, evidence-based sort of person and/or society — from an imbalance of the four humours of the body. The four-humours explanation made sense. It was practical and workable. It led to treatment. It was wrong in every respect. Some progress has been made since then, as I’m sure you’ve noticed. You’ll find out a bit about that progress later on, but for the time being it’s enough to say that humanity now has at least a partial understanding of the mechanisms and causes of disease — and it’s turned out to be not very simple at all. If a scholar of yore had been able to read a modern medical textbook, what he would in all probability have been most struck by is how ridiculously, bewilderingly complicated health and disease are now understood to be. Demons, divine will, or an excess of bile have been replaced with the wonderful world of bacteria and viruses, toxins and free radicals, leukocytes and antigens and antibodies, cytokines and chemokines, MHC molecules and V(D)J recombination and hypervariable antigen binding and CD25+ regulatory T-cells and … It’s enough to make anyone’s head spin.

To make matters worse, diseases can be genetic, or infectious, or can be the result of the body’s own workings breaking down in one way or another. Most diseases are caused by a combination of any of the above. For instance: you can’t catch cancer from other people — except for the types that you can (which I’ll talk about in chapter five). Or: you get infected with malaria by mosquito bites — unless you’re naturally immune to it by virtue of a certain allele of your DNA. And so on. The more we find out, the less well defined it all seems to be.

And why, our hypothetical ancient scholar reading through the descriptions in a modern textbook would wonder, would Nature operate in so convoluted a way as to have a human  disease, caused by an invisible organism, pass through yet another organism — in some cases, two other organisms — en route between one human and another? What sense does it all make?

‘Nothing in biology makes sense except in light of evolution’, wrote Theodosius Dobzhansky in a famous essay. Charles Darwin provided us with the basis for the only satisfying answer we have for the overwhelming complexity of the natural world, and so immunologists have been applying the Darwinian perspective to their field in order to understand why the immune system looks and operates the way it does. I’ll get to that later on.

In the meantime, I have a problem. It’s a problem I share with any writer who wishes to drive home the point that something is complicated. Simply saying ‘It’s complicated’ not only doesn’t really convey any of the flavour, but it also sounds sort of lazy. On the other hand, this book is meant to be read by you — the interested layperson or student. It’s not a  textbook, and so while laying out the complications in agonising detail would indeed make the point, the reader would suffer for it, and readers don’t tolerate this kind of behaviour anymore; I might find myself unceremoniously tossed back on the bookshelf, and it’s cramped up there.

How, then, should I say how complicated the immune system is?