fits in with the carriage of the infection in the same way from man to man, and not with the notion of an aerial dispersion of the infection. The fact that typhus only exists in very dirty and crowded populations, and that it has disappeared where even a moderate amount of cleanliness as to person and clothing has become general, coincides with the possibility of the body louse as carrier. This little parasite is known to be a wanderer, and is gifted with a very acute sense of smell. An individual placed in the centre of a glass table invariably walked, guided by the scent, towards the observer, at whatever position he placed himself. Sulphurous acid is a violent repellant of these creatures. Not only will it kill them if they are exposed to its fumes, but traces of it drive them away. Hence doctors and nurses who have to handle typhus patients or their clothes have only to wear a small muslin bag of sulphur under their garments, or to rub themselves with a little sulphur ointment in order to be perfectly guarded against infection; the louse will not approach them, nor remain upon them should it accidentally effect a lodgment. It is not always obvious at once in what way a knowledge of the mode of carriage of a deadly disease can be of service to humanity. But in this case it is strikingly and triumphantly clear. In the vast poverty-stricken population of Russia typhus is still common. Public medical officials attend these cases, and the Russian Government keeps a record of the annual deaths of its medical staff, and of the causes of their deaths. In the first six months of last year 530 Russian medical officers died, and twenty-four of these deaths were caused by typhus fever acquired by these devoted public servants in attendance upon cases of that fever. Henceforth they will make use of sulphur or sulphurous ointment to keep the little infection-carriers at a distance, and not one medical man or nurse will catch the disease, still less be killed by it. A remarkable fact in this history is that the actual parasitic germ which causes typhus, whether a bacterium (Schizophyte) or a protozoon, has not been detected, although the louse has been shown to be its "carrier." The same is true of yellow-fever: we have not seen with the microscope the microbe which produces it. But we know with certainty that the gnat, _Stegomya fasciata_, and no other, is the carrier of the unseen germ, and that we can obliterate that fever by obliterating the gnat. So, too, although we know how the infection of rabies acts, and how it is carried, yet no one has yet isolated and recognised the terrible infective particle itself. There is a very high probability that in these cases, and also in cancer (where as yet no specific infective germ or parasitic microbe has been detected), such an infective microbe is nevertheless present, and has hitherto escaped observation with the microscope on account of its excessive minuteness and transparency. CHAPTER XXI