This is a book about germs, known to scientists as microbes (or to some, who cannot use a short word where a long one exists, as microorganisms). These creatures, which are largely invisible, inhabit every place on Earth where larger living creatures exist; they also inhabit many parts of the Earth where no other kinds of organism can survive for long. Wherever, in fact, terrestrial life exists there will be microbes. Conversely, the most extreme conditions that microbes can tolerate represent the limits within which life as we know it can exist.

The biosphere is the name biologists give to the sort of skin on the surface of this planet that is inhabitable by living organisms. Most land creatures occupy only the interface between the atmosphere and the land; birds extend their range for a couple of hundred metres into the atmosphere, burrowing invertebrates such as earthworms and nematodes may reach a few metres into the soil but rarely penetrate further unless it has been recently disturbed by man. Fish cover a wider range, from just beneath the surface of the sea to those depths of two or more kilometres inhabited by specialized, often luminous, creatures. Spores of fungi and bacteria are plentiful in the atmosphere to a height of about a kilometre, blown there by winds from the lower air. Balloon exploration of the stratosphere as long ago as 1936 indicated that moulds and bacteria could be found at greater heights; more recently the USA's National Aeronautics and Space Administration has detected them, in decreasing numbers, at heights up to thirty-two kilometres.