As regards diphtheria, the chief causes of the spread of this disease are mistaken diagnosis, imperfect isolation, incomplete disinfection, and, paradoxical though it may seem, a lack of susceptibility to the disease in a large number of children. Many physicians are still under the grave error that diphtheria can always be recognised without the aid of the microscope, and that membranous croup commonly kills. All scientific writers upon diphtheria agree that it is caused by the Klebs-Loeffler bacillus. They also hold that there is a disease called membranous croup, as distinct from diphtheria as typhoid is, but that membranous croup is a comparatively harmless and non-contagious disease. Two per centum is a liberal mortality in membranous croup, yet a certain class of physicians are constantly reporting deaths from this disease. In a series of 286 cases (not deaths) diagnosed as membranous croup by physicians of New York {188} City a few years ago, Park found the diphtheria bacillus in 229, or 80 per centum. I have never examined the throat of a child dead from so-called membranous croup in which I did not find the diphtheria bacillus. This is the experience of almost every bacteriologist who has had to do with diphtheria. Some men report deaths from diphtheria as thrush! These deaths might just as truthfully be attributed to the wearing of linen collars. On the other hand, according to Baginsky of Berlin, Martin of Paris, Park of New York, and Morse of Boston, from 20 to 50 per centum of the cases admitted even to diphtheria hospitals have not diphtheria at all. Bacteriologists find that about 35 per centum of the cases reported by physicians to be diphtheria are really nothing but tonsilitis or pharyngitis, with now and then a case of membranous croup. Without a bacteriological diagnosis, therefore, 35 families in each 100 quarantined (where quarantine laws exist) are unjustly quarantined and subjected to the trouble and expense of useless disinfection. The suffering this can cause to a poor family, whose small business is often ruined by quarantine, is a matter for very serious consideration. Again, no matter what experience a physician may have had, he can not in many cases differentiate diphtheria in its early stages, or in children of good resisting power, from comparatively harmless throat affections. The extraordinary resisting power against diphtheria shown by some children and adults has been described by Wassermann (_Zeitschrift f. Hyg._, 19 B., 3 H.). He found one series of 17 children, from one and a half to eleven years of age, and 34 adults, in which 11 children and 28 adults were not only immune to diphtheria, but some of them had enough antitoxin in their blood to neutralise a tenfold fatal dose of diphtheria toxin. This explains many mysterious outbreaks of diphtheria: such immune persons are infected and they carry about the disease unconsciously because they are not ill themselves.