above, the incubation period of yellow fever— the period elapsing between the introduction of the virus into the body and the on-coming of fever— is usually only from three to five days, yet a period of at least twelve days must elapse before that virus, after removal from one human body, can be effectively implanted in another human body. What becomes of it during these twelve days? The answer to this question has now been supplied, and with it the key to most of the epidemiological peculiarities of yellow fever.
Duration of the infective period.— This is singularly brief. A yellow-fever patient is dangerous to his neighbours only during the first three or four days of the disease. After this he cannot confer infection.
The germ.— A great many attempts have been made to discover the germ or virus of yellow fever, and a corresponding variety of organisms have been described. In the belief that some of these were the true germs, certain bacteriologists went the length of practising protective inoculation with attenuated cultures of their respective organisms, whilst others elaborated curative sera on the orthodox lines. Both sets of enthusiasts have claimed successes. We now know, however, that the germ of yellow fever is not a bacterium, at all events is not a visible bacterium, and that it cannot be cultivated or modified on the ordinary bacteriological lines. Freire's Cryptococcus xanthogenicus, Finlay's Tetragenus febris flava, and Sanarelli's Bacillus icteroides have no longer anything but an historical interest.
At the instigation of the Government of the United States, these and other organisms were rigidly investigated by Sternberg, whose standing as a bacteriologist and whose judicial cast of mind eminently qualified him for forming a trustworthy judgment. His verdict on them, and on many similar organisms for which pathogenic claims had been advanced, was unfavourable.
Seidelin has recently described certain very minute and somewhat indefinite supposed bodies in the red
