The specific organism of malaria may become active again and again in the blood, causing relapses twenty years or more after the original infection. The specific organism of yellow fever expends itself at once, in one acute attack; and, if the patient recovers, he is thenceforth more or less immune against infection. That the inoculation of the disease, by the application of a single mosquito recently contaminated, is calculated to produce a mild or abortive attack less dangerous than the average attack among the non-acclimatised, was known to Finlay, and was confirmed in 1899 by the Army Commission of the United States.
Of the mortality of the disease, Sir Patrick Manson, in 1900, wrote as follows:—
"It is better for women and children than for men; better for old residents than for newcomers; worst of all for the intemperate. According to a table of 293 carefully observed cases given by Sternberg, the mean mortality in the whole 293 cases was 27.7 per cent. This may be taken as a fairly representative mortality in yellow fever among the unacclimatised, something between 25 and 30 per cent., although in some epidemics it has risen as high as 50 or even 80 per cent. of those attacked.... Some of these epidemic visitations bring a heavy death-bill; thus, in New Orleans, in 1853, 7970 people died of yellow fever; in 1867, 3093; in Rio, in 1850, it claimed 4160 victims; in 1852, 1943; and in 1886, 1397. In Havana, the annual mortality from this cause ranges from 500 to 1600 or over."
The earlier attempts to reproduce the disease, by inoculation with its products, failed altogether:—
"In 1816, Dr. Chervin, of Point-à-Pitre (Antilles), drank repeatedly large quantities of black vomit without feeling the least disturbance. Some years before, other North American colleagues, Doctors Potter, Firth, Catteral, and Parker, did everything possible to inoculate themselves with yellow fever. After having uselessly attempted experiments on animals, they experimented on themselves, inoculating the black matter at the very moment in which the moribund patient rejected it, placing this matter in their eyes, or in wounds made in their arms, injecting it more than twenty times in various parts of their body ... in short, devising every sort of daring means for experimentally transmitting yellow fever. All these experiments were without result, and in the United States during many years it was believed that this terrible malady was non-contagious." (British Medical Journal, 3rd July, 1897.)
The history of the subject, from 1812 to 1880, is given by Dr. Finlay of Havana, in the New York Medical Record (9th February 1901). In 1880, two very important reports on the disease were published; one by a Havana Commission of the National Board of Health of the United States, the other by the United States Navy Department. They tended to show that yellow fever is a "germ-disease"; that it is not wind-borne; and that there may be some change, outside the body of the patient, whereby the virulence of the active principle of the disease is heightened. From these reports, Dr. Finlay advanced his doctrine that the mosquito receives and transmits the germs of the disease:—
"It was upon the above line of reasoning (in these reports), that I conceived the idea that the yellow-fever germ must be conveyed from the patient to the non-immunes by inoculation, a process which could be performed in nature only through the agency of some stinging insect whose biological conditions must be identical with those which were known to favour the transmissibility of the disease."
In 1881 he inoculated himself and six soldiers with infected mosquitoes, and obtained, as he had calculated, mild attacks and subsequent immunity. During the years 1881-1900 he inoculated by this method 104 persons:—
"In these inoculations, be it remembered, my principal object was rather to avoid than to seek the development of a severe attack; in point of fact, only seventeen showed any appreciable pathogenic effects after their inoculation. I felt sure, however, that severe or fatal result might follow an inoculation either with several mosquitoes contaminated from severe cases of the disease, or from a single insect applied several days or weeks after its contamination, having come to this last conclusion in view of the facts connected with the Anne Marie, and the epidemic of Saint Nazaire."
Dr. Finlay's discovery that the mosquito can convey yellow fever, and that the germ of the disease is more virulent after a prolonged sojourning in the mosquito, was proved beyond all question by the work of 1889-1901. But, so far as immunisation is concerned, few people would submit themselves to be bitten by an infected mosquito, even with perfect assurance that the germs contained in it were of a low degree of virulence: the urgent need, therefore, was for an immunising serum. In 1896, at Flores, Sanarelli discovered the bacillus icteroides; and by October 1897, he had prepared an immunising serum which was able to give a considerable amount of protection to animals.[40] Next year (Annales de l'Institut Pasteur, May 1898) came the news that he had advanced against yellow fever with its own weapons—Premières expériences sur l'emploi du sérum curatif et préventif de la fièvre jaune. Of the first 8 cases (Rio de Janeiro), 4 recovered. Then came the 22 cases at San Carlos do Pinhal, in Saint-Paul au Brésil (January 1898), with 16 recoveries, and only 6 deaths. And it is to be noted that he submitted his method of treatment to the utmost test that was possible; he chose the bad cases, and the country where the fever was most fatal:—