What’s a man? Or, indeed, a woman? Biologically, the answer might seem obvious. A human being is a(n)【C1】_____who has grown from a fertilised egg which【C2】__genes from both father and mother. A growing band of biologists,【C3】__. think this definition incomplete. They【C4】__people not just as individuals, but also as ecosystems. In their view, the descendant of the fertilised egg is【C5】__one component of the system. The others are trillions of bacteria, each equally an individual,【C6】_____are found in a person’s gut, his mouth, his skin and all of the crevices and orifices that exist in his body’s surface.

A【C7】_____adult human harbours some 100 trillion bacteria in his gut alone. That is ten times as many bacterial cells as he has cells【C8】__from the sperm and egg of his parents. These bugs, moreover, are【C9】__. Egg and sperm provide about 23,000 different genes. The microbiome, as the body’s commensal bacteria are collectively known, is reckoned to have around 3 million.【C10】__, many of those millions are variations on common themes, but equally many are not, and even the number of【C11】_____that are adds something to the body’s genetic mix.

And it really is a system, for evolution has【C12】_____the interests of host and bugs. In exchange for raw materials and shelter the microbes that live in and【C13】__people feed and protect their hosts, and are thus integral to that host’s well-being. Neither wishes the other harm. In bad【C14】__, though, this alignment of interest can【C15】_____. Then, the microbiome may misbehave in ways which cause disease.

That bacteria can cause disease is no【C16】_____. But the diseases in question are. Often, they are not acute infections of the【C17】__20th-century medicine has been so good at dealing with. They are, rather, the chronic illnesses that are now,【C18】__in the rich world, the main focus of medical attention.【C19】__, from obesity and diabetes, via heart disease and asthma to neurological conditions such as autism, the microbiome seems to play a(n)【C20】_____role.

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