Whether it be an intellectual Dorothy Sayers, a clever Agatha Christie, the latest psychological Simenon—or for that matter, the article on page 1849 of this issue of The Journal—the ele ments of a good whodunit are much the same. For, like the best fictional mystery stories, the account of the epidemiological investigation by Rosenberg, Hazlet, and Schaefer could not have been better plotted. The place is the Mississippi River, along the bluffs below Dubuque; the time, summer 1974, just before school begins. The characters are some 45 youngsters, aged 9 years and thereabouts who have been attacked by a member of the Shigella family: the villainous sonnei. The villain in turn is being stalked by detectives, in this case, the authors themselves, supported by a large force of state, local, and national health authorities. But the case is baffling. The villain has apparently changed his MO; never before has