Further, statistical data, when investigated and scrutinized by exact methods and correlated with clinical observations, may yield valuable information regarding the nature and origin of disease. If a malady is found to be periodic, the climatic and physical changes associated with the rise and fall of the infection may indicate the causal agents, and when their identity is established effective measures may be taken against them. Rogers' researches show that cholera begins to spread only when the degree of atmos pheric humidity has reached a certain figure ; in consequence, the possible commencement of an epidemic can be foreseen and appro priate prophylactic measures undertaken in advance. Already trop ical research has achieved great triumphs, and when the true economy of preventive medicine is realized more fully in tropical administration, the return in increased efficiency will multiply.
Results.—Bef ore the last decade of the i9th century, the agent responsible for the spread of malaria (q.v.) was unknown. No eye had seen the bacillus of plague ; and fleas were thought merely unpleasant creatures, rather less scandalous than lice. Yellow fever mosquitoes besported themselves unheeded, except in so far as they might disturb an afternoon's siesta. But when Patrick Manson argued that some biting insect serves as intervenient host for the nematode parasite of man, Filaria bancrofti, and turned his theory into proven fact, he stood like Cortez on a peak in Darien, viewing a new world. Following on Manson's example and advice, and faced by difficulties which would have disheart ened most, Ross carried out his laborious researches and finally Anopheles was incriminated as the vector of malaria.
In the old yellow fever (q.v.) days, a king's ship, the Tiger cruised off Barbados, and out of a complement of 220, buried 600 men dead of yellow fever within two years ; the commander, as he reported, "still pressing men out of the merchant ships that came in, to recruit his number in the room of those that died daily." The heroic work of Reed and his collaborators proved that this infection is transmitted through the bite of a mosquito, Aedes.argenteus (Stegomyia fasciata), and Noguchi's indication of a causative micro-organism apparently confirmed their work. For tunately the peculiar breeding habits of the insect vector render it relatively easy to control, and in the West Indies to-day yellow fever is little more dreaded than in England.
Prior to recognition of the curative properties of intravenous antimony, kala azar (q.v.), which decimates rich populous tracts of Bengal and Assam, killed over 9o% of those attacked. The disease is in process of investigation by a special commission, and the mode of transmission, so far baffling identification, is likely to be revealed. If the active agent proves to be a species of "sand fly," as has been surmised, the success of the hygienic methods adopted in practice will be explained.
The progress of physiological knowledge in the special branch of endocrinology has secured success in the treatment of cases of sprue (q.v.), where the restoration of the calcium content of the blood has been effected by the administration of parathyroid extract.
Cholera and plague may be stayed by prophylactic vaccines, and the results of anti-typhoid inoculation were one of the medical triumphs of the World War. The incidence of tropical liver abscess has been enormously reduced by the use of emetine, and the special "liver-abscess-days"—earmarked for surgical opera tions—once a feature of some tropical hospitals, are now a thing of the past. Synthetic chemistry adds to the resources of the scientific physician of tropical diseases; "Bayer 205" and trypar samide in the treatment of African sleeping sickness have given hopeful results, while new antimony compounds are replacing tartar emetic for intravenous medication, seemingly with all its advantages and none of its drawbacks. Thus the diseases which take so heavy a toll of human life are one by one yielding to science and coming under control, and the outlook to-day is brighter than even the most optimistic would have dared to prophesy some thirty years ago. In honouring the men who thus have extended the science of medicine, we should not forget those of old time who caught glimpses of truth, often through a glass darkly, but at times with a clearer vision.