THE MIGRATIONS OF MALARIA 271 



contagious maladies pass from one individual to another renders 

 their diffusion through a population comparatively slow. They 

 are endemic from the first. They rarely cause great epidemics ; 

 but, once established, are never again absent from their new habitat. 



449. The microbes of all other diseases alternate between the 

 human body and the world outside, in which, on the way to fresh 

 ' hosts/ they pass a longer or a shorter time. They are depen- 

 dent, therefore, to a greater or lesser extent on this outer environ- 

 ment. The insect-borne maladies are found only in localities 

 where the insect host is able to maintain existence. Malaria was 

 once common in England, but the conditions became unfavourable 

 to the mosquito, and the disease disappeared. Though many 

 infected people from the tropics land on our shores, it cannot re- 

 establish itself. In historic times it is said to have invaded 

 Greece and Italy, and by some medical writers is supposed to 

 have contributed to the downfall of the classic empires. In more 

 modern times it invaded Mauritius. The fact that before the 

 historical era malaria had spread to most parts of the globe where 

 its intermediate host, the Anopheles mosquito, is capable of per- 

 sisting, is evidence of its vast antiquity. Of all the host of 

 maladies which now infest the Western Hemisphere it is apparently 

 the only one of any importance which was not introduced by 

 Columbus and his successors. Possibly it is the most ancient of 

 all human diseases. The path by which it journeyed to America 

 is quite unknown. Under present conditions of climate it can 

 hardly have travelled by the Arctic circle. Before the era of 

 European discovery man had crossed the Pacific, all the large 

 islands of which are thickly populated. But, except near 

 Asia, malaria is unknown in them. The difficulties of crossing the 

 practically uubroken expanse of the Atlantic are obvious. The 

 antiquity of the disease is evidently so great that the surmise that 

 it may have reached America during a warm inter-glacial period is 

 perhaps not altogether extravagant 



450. Plague, sleeping sickness, and yellow fever are other very 

 important insect-borne maladies. Plague is a disease of rats, the 

 intermediate host being a species of flea which is parasitic on these 

 animals, but which will on occasion attack human beings. During 

 an epidemic the rats tend to be exterminated and the disease to 

 die out. It is endemic in India, but several great pandemics in 

 which it spread widely are recorded in history. It was known in 

 Europe before the Christian era. 1 In the sixth century after 

 1 Hksch, vol. i. p. 494. 



