A resoundingly fun if sometimes unappetizing inquiry into the world of microbes, germs, and other invisible but influential phenomena. Popular-science writer Biddle (A Field Guide to Germs, 1995) here offers an Allergens-to-Zeitgeist series of smallish essays, ""necessarily severe distillations,"" about the things that lie beyond our immediate senses and are thus not only out of sight but out of mind. He delights in turning up little facts that make fine fuel for did-you-know party conversation: The average adult, for instance, breathes 13.5 kilograms of air each day, about four times more in weight than the food and water he or she ingests; if all photosynthesis were to cease tomorrow, the Earth would still have an 8,000-year supply of oxygen; if you turn your back to the wind, low pressure will always be to your left side, high pressure to your right; a scream can indeed make someone's ""blood run cold,"" inasmuch as loud noises can lower blood pressure and heart rate. Along the way he addresses such matters of perennial interest as farts, burps, and cooties (the first the product of ""nefarious methanogens,"" the last the horrible bearers of typhus, to say nothing of schoolboy terrors), and he urges that such things be not too much feared; as he writes in the instance of bad breath, ""bacteria . . . cannot be escaped, household disinfectants notwithstanding. . . . Nothing is more natural than meeting a microbe. It's their scene."" It's their scene indeed, and Biddle does a fine job of making it meaningful to his readers. Biddle's book adds up to little more than an assemblage of scientific and cultural factoids and gross-out trivia--which makes it just right for bright teenagers of an inquiring bent, and for collectors of useless information everywhere.