48 



several reasons it seems to me not improbable that this may 

 have happened in the plague of the Middle Ages. 



OTHER DISEASES TRANSMITTED BY INSECTS. 



Three other diseases which can be transmitted by insects 

 typhus, relapsing fever, and poliomyelitis must be men- 

 tioned, although I am not sure*fchat I am justified in including 

 any of them under the heading "bacterial." The infective 

 agent in typhus has so far not been isolated, but it is ascer- 

 tained that it is an organism capable of propagation, for 

 Nicolle (1910) and Wilder (1911) have succeeded in carrying 

 the disease through a number of generations of monkeys by 

 inoculating them with 1 to 5 c.c. of blood or serum from an 

 infected animal. The organism is held back by Berkefeld 

 and porcelain filters. It is therefore free in the blood and of 

 some considerable size. 



Relapsing fever is caused by a spirochgete, and there is 

 considerable difference of opinion as to whether spirochsetes 

 should be classed amongst the bacteria or protozoa. 



The infective agent of poliomyelitis is a minute invisible 

 organism which passes readily through a porcelain filter, so 

 that it escapes the clutches of the systematist altogether. 



THE TRANSMISSION OF TYPHUS FEVER BY LICE. 



Typhus is a disease associated with poverty and dirtiness. 

 It occurs in temperate climates and mostly in the winter and 

 spring. In warm climates it is uncommon. In Mexico, 

 where it still flourishes amongst the poor Indians and half- 

 caste population, it is confined to the central plateau 4000 to 

 10,000 feet above the level of the sea, and does not affect the 

 population of the lower hot country, although cases are 

 frequently introduced from the higher lands. 



Typhus has been shown to be capable of transmission by 

 the agency of body lice, Pediculus vestimenti, by Nicolle, 

 Compte, and Conseil (1909), and by Ricketts and Wilder 

 (Wilder, 1911). 



The Bionomics of Lice and the Mechanism by means of which 

 they might Carry Infection. 



The life-history of the body louse has recently been studied 

 by Warburton (1909), to whom and to my colleague, Mr. Bacot, 

 I am indebted for most of the following facts on the subject. 

 Unlike fleas, lice have no grub stage. The eggs take one to 

 five weeks or longer to incubate, according to temperature. 

 The young louse when just hatched is a small edition of the 

 parent and sucks blood at the earliest opportunity. It is, 



