THE PRESIDENT'S ADDRESS. 445 



important facts relating to the transmission of yellow fever 

 were elicited. It was shown that the unknown cause of the 

 disease is in the blood of the patient only during the tirst three 

 days of the illness, so that only during this period can mosquitoes 

 become infected by sucking the blood of the patient. Conse- 

 quently if the patient be protected from mosquitoes for the first 

 three days he ceases to be a danger to the community as a source 

 from which the infection can spread. It was shown further that 

 the mosquito, after acquiring the infection, goes through an 

 incubation period of from twelve to fourteen days, during which 

 it is not infectious ; but after that it is infectious for the rest of 

 its natural life. And a further point of interest was added by 

 the French commission — namely, that an infected mosquito may 

 transmit the infection to its offspring, so that a mosquito which 

 has never fed on an infected person maybe congenitally infectious. 



I have chosen the instance of yellow fever to put before you 

 because, although we have now such an accurate knowledge, 

 gained by experiment, of the cause and transmission of the 

 disease, no one has succeeded as yet in seeing the parasite 

 itself. It is practically certain for many reasons that there 

 is some minute parasite at work, and there are grounds for 

 suspecting that the parasite is a spirochaete, one of those minute, 

 actively flexible, thread-like organisms of which the affinities 

 are so much in dispute at present, and which some authorities 

 class with the Protozoa, others with the Bacteria. But here we 

 have a case where the microscopist has been baffled, and where 

 we get beyond the present limits of the powers of our instrument, 

 a fact which should make us appreciate the labours of those who 

 study the microscope and strive to perfect it. 



Did time permit, I might mention many more important 

 discoveries in the field of Protozoan parasites causing disease. 

 For example, there are the blood parasites of the genus Piro- 

 plasma (Babesia), causing fatal forms of haemoglobinuria in 

 various animals ; they are not yet known for certain in man, 

 but a species is known from monkeys, a source which is getting 

 perilously near to us. Here the agent of infection is a tick of 

 some sort, and usually the infection goes through two generations 

 of ticks, being transmitted from the mother-tick, which has 

 acquired the infection, to the numerous progeny of minute six- 

 legged tick-larvae, which in their turn infect the vertebrate host. 



