Novembers, 19 19] 



NATURE 



21 1 



obvious cases of the malady, and the transmission 

 of the infective agent from one individual to 

 another. 



If the infective agent is present in a super- 

 ficial lesion, as in smallpox, syphilis, diphtheria, 

 or pneumonia, or passes out with the excreta, as 

 in cholera and typhoid fever, more or less direct 

 transmission can occur, but in the case of a para- 

 site situated only in the blood or internal organs 

 it was for long a mystery how the disease was 

 transmitted. The secret was revealed by the dis- 

 coveries of Manson, Smith, and Bruce on 

 filariasis, red-water fever, and Nagana, showing 

 that in these diseases mosquitoes, ticks, and 

 tsetse-flies respectively acted as transmitters. 

 These observations were soon followed by those of 

 Ross on the transmission by mosquitoes of malaria, 

 and afterwards it was shown by the American 

 Commission that yellow fever also was trans- 

 mitted by a particular species of mosquito. 



Relapsing fever, sleeping sickness, and bubonic 

 plague were also found to be spread by the agency 

 of insects ; ticks or lice in the first case, a 

 tsetse-fly in the second, and fleas in the last, and 

 the most recent addition to the list is trench fever, 

 which has been proved to be louse-borne. 



The dependence of these maladies for their dis- 

 semination upon particular species of insects has 

 afforded a long-looked-for explanation of their 

 distribution — e.g. sleeping sickness, yellow fever, 

 and dengue — and the very extensive investiga- 

 tions into the life-history of the parasites and their 

 insect hosts has enabled the sanitarian to choose 

 the stage in the cycle most convenient for attack. 

 He could strike at the enemy whilst it v/as resi- 

 dent in either host or indirectly by preventing the 

 insect from biting the patient and other individ- 

 uals until in course of time the infection died out. 

 By netting-in patients suffering from yellow fever 

 so that mosquitoes could not attack them, and 

 at the same time insisting on the removal of all 

 small collections of water in the neighbourhood of 

 habitations in which these insects were wont to 

 lay their eggs, Gorgas rid the city of Havana of 

 yellow fever. By a campaign on similar lines 

 against malaria-bearing species of mosc|uitoes, the 

 Isthmus of Panama was converted into a health 

 resort. Equally satisfactory results have followed 

 elsewhere when it has been possible to institute 

 equally thorough measures. 



Before leaving the subject of Infection, I must 

 not omit to mention that biological discoveries 

 regarding the life-history of the parasitic worms 

 - — e.g. the hookworms and Bilharzia — have shown 

 how diseases caused by this class of parasites 

 could be successfully controlled. 



It has not often been found possible to eliminate 

 the cause of a disease. In some cases knowledge' 

 has not been sufficiently complete. In others its 

 application has been too difficult, and it has been 

 found impracticable sufficiently to control the 

 lives of the population. In many such cases, 

 however, preventive medicine has another 

 NO. 2610, VOL. 104] 



arrow in her quiver. This is aimed at reducing 

 the susceptibility of a population to a particular 

 infection by protective inoculation. The earliest 

 effort of preventive medicine along these lines 

 was that of inoculation against smallpox prac- 

 tised in Asia for some centuries and introduced 

 into England in 1721 by Lady Mary Montagu. 

 Cutaneous inoculation of smallpox usually pro- 

 duces a local and comparatively mild illness, but 

 the method suffers from the disadvantage that it 

 propagates the virus of the disease. Jenner's 

 vaccination with cow-pox — a modified virus — 

 obviated this disadvantage. 



With the discovery of the microbial origin of 

 disease, Pasteur saw that the principle of Jen- 

 nerian vaccination might be further exploited, 

 and in 1881 successfully employed attenuated cul- 

 tures of the microbes of splenic fever and chicken 

 cholera to protect flocks and poultry against the 

 depredations of these diseases. 



In the case of man, the possible danger from 

 the employment of living cultures of the germs 

 of fatal diseases led to researches to determine 

 whether the injection of the microbes which had 

 been killed by heat or chemical agents also in- 

 duced some measure of protection against a sub- 

 sequent inoculation with living virulent organisms. 

 By experiments on animals this was found to be 

 the case, and the use of such bacteria! "vac- 

 cines " was employed by Haffkine to protect man 

 against cholera and plague. Shortly afterwards 

 Wright and Semple elaborated a similar method 

 of protective inoculation against typhoid fever. 

 Anti-typhoid inoculation has been extensively 

 used. The experience in the British and Ameri- 

 can -Armies during the last fifteen years has been 

 that a material reduction in the incidence of the 

 disease has occurred amongst inoculated troops. 



The greatest triumph of preventive medicine 

 during the late war was the comparative rarity 

 of typhoid fever amongst our troops. This was 

 the case not only in France, but also in military 

 operations in other areas, where the conditions 

 were such that satisfactory hygienic measures 

 could not be carried out. No other explanation 

 of this freedom from enteric is forthcoming other 

 than the periodic prophylactic inoculations to 

 which our armies were subjected. 



So far I have dealt exclusively with infection 

 by living patViogenic agents. I make no apology 

 for so doing, for the great developments in pre- 

 ventive medicine throughout the world which are 

 characteristic of this period have been due to the 

 impetus given by the conceptions of Pasteur and 

 the methods of Koch. 



At the same time, knowledge in all departments 

 of physiology and pathology has steadily, though 

 less dramatically, progressed. The increased 

 understanding of animal nutrition must, owing to 

 its important bearing upon the maintenance of the 

 health of the peoples, be briefly refeired to. 



Before the period under review Pettenkofer and 

 Voit had been able to strike a balance-sheet of the 



