272 EPIDEMIC AND ENDEMIC DISEASE 



Christ it " spread over the whole Roman Empire of the East and 

 West and even far beyond the limits of the empire ; . . . and 

 wrought the most frightful devastation wherever it appeared ; . . . 

 it depopulated towns, turned the country into a desert, and made 

 the habitations of men to become the haunts of wild beasts." 1 

 Another great pandemic, " known everywhere under the name of 

 the Black Death as one of the great events in the world's history," 

 " reached over the whole of the known world " and destroyed a 

 third or a quarter of the inhabitants of Europe. The Great Plague 

 of London furnishes an additional example. 



451. Yellow fever, like malaria, has the mosquito for its inter- 

 mediate host. Its original habitat appears to have been the West 

 Coast of Africa, but it is now endemic in the tropical parts of 

 Eastern America. Till very recently sleeping sickness appears to 

 have been confined to the West Coast of Africa. It was often 

 carried by slaves to Brazil and the West Indies, where, however, in 

 the absence of its intermediate host the tsetse fly (Glossina 

 palpalis\ it did not spread. The infected slaves were supposed 

 to be suffering from nostalgia or home-sickness. 2 Owing to the 

 improvements of communications consequent on European con- 

 quest, it has spread very widely and has now almost reached the 

 East Coast near Zanzibar, where it is likely to prove a scourge as 

 terrible as it is already in Uganda. Doubtless the tsetse fly 

 preceded the disease in Central Africa, but did no great harm till 

 infected human beings arrived. The insect-borne diseases are the 

 only complaints which, on the whole, afflict sparse populations 

 more than they do denser communities of men. 



452. Air-borne maladies are immensely infective but of short 

 duration in the individual. After a brief illness the sufferer dies or 

 acquires an immunity which may or may not be permanent. Con- 

 sequently, in small and isolated communities, they can occur only 

 in epidemic form, at any rate when the immunity they confer is at 

 all lasting. Owing to the large number of susceptible persons, 

 such a disease, when it reaches a new country, spreads like fire. 

 If it has long been absent, it strikes down the whole population. 

 So many are infected, so abundant do the microbes become, that 

 no one escapes the chance of infection. As a consequence, the 

 food-supply of the microbes is swiftly exhausted, and the malady 

 disappears till re-introduced from foreign sources. On the other 



1 Hirsch, vol. i. pp. 495-6. 



2 The Kingdom of Man, by Sir E. Ray Lankester, p. 159 (London : Constable 

 & Co., 1907). 



