IS IT DANGEROUS TO BE too clean? “Clearly, if true, the hygiene hypothesis is the single greatest medical story of our time, undercutting a century of putative progress. Is it true? Probably some of it is. But Mr. Velasquez-Manoff’s ambitious compendium of data and supposition — a great dense fruitcake of a book whose 680 endnotes, the author notes apologetically, refer to only a minority of the 10,000 studies he consulted — spins it all out in the most positive possible way with an energy, eloquence and desire to believe that is both breathtaking and a little scary. . . . Enthusiastic patients, however, enabled by the usual cast of shady entrepreneurs, have predictably skipped the long, dull validation process and are busily infecting themselves with a variety of intestinal worms.”