Never fall in love. Oh, well, go ahead, if you must, but make sure you fall for a living being. A romance with anything else, be it a pair of shoes or a scientific theory, is only asking for trouble, so seldom will the inanimate beloved live up to your besotted expectations.

The best scientists struggle with this prohibition daily, knowing that even the most seductive data may well disappoint. But premature enthusiasm is routine among others, patients and their doctors foremost among them, with journalists smelling a scoop not far behind.

Moises Velasquez-Manoff — a journalist and also, as it happens, a patient — has fallen hard for an idea known as the hygiene hypothesis, whose implications, if followed out along a widely branching chain of extended supposition, threaten to unravel much of what we think we know about health and disease.

This hypothesis argues that our modern obsession with eradicating germs has backfired into an explosion of disease, specifically all the “new” diseases that have replaced infections to undermine our health. The modern immune system, the idea holds, is stymied by the sudden absence of its customary microbial targets. With nothing constructive to do, it is crazily spinning its wheels, resulting in soaring rates of food allergies and asthma, arthritis, psoriasis, multiple sclerosis and diabetes, even heart disease and cancer — not to mention alopecia, the premature baldness from which Mr. Velasquez-Manoff suffers and which led him to the subject in the first place. (In an opinion article in The New York Times last month, he suggested that an immune disorder might account for many cases of autism.)