PLAGUE. 435 



The evidence, accordingly, that the influenza bacillus is the 

 cause of the disease rests chiefly on the well-established fact 

 that it is always present in the secretions of the respiratory 

 tract in true cases of influenza, and that it is an organism which 

 has not been found in any other condition. Moreover, it is an 

 organism which is practically restricted by its conditions of 

 growth to the animal body. A certain amount of confirmatory 

 evidence has been supplied by the results of experiment. 



Methods of Examination. — («) Microscopic. — A portion of 

 the greenish-yellow purulent material which often occurs in 

 little round masses in the sputum should be selected, and film 

 preparations should be made in the usual way. Films are best 

 stained by Ziehl-Neelsen carbol-fuchsin diluted with ten parts 

 of water, the films being stained for ten minutes at least. In 

 sections of the tissues, such as the lungs, the bacilli are best 

 brought out, according to Pfeiffer, by staining with the same 

 solution as above for half an hour. The sections are then 

 placed in alcohol containing a few drops of acetic acid, in 

 which they are dehydrated and sHghtly decolorised at the same 

 time. They should be allowed to remain till they have a mod- 

 erately light colour, the time varying according to their appear- 

 ance. They are then washed in pure alcohol, cleared in xylol, 

 and afterwards mounted in balsam. 



(b) Ctdttires. — A suitable portion of the greenish-yellow 

 material having been selected from the sputum, it should be 

 washed well in several changes of sterilised water. A portion 

 should then be taken on a platinum needle, and successive 

 strokes made on the surface of blood-agar tubes. The tubes 

 should then be incubated at 37° C, when the transparent 

 colonies of the influenza bacillus will appear, usually within 

 twenty-four hours. These should give a negative result on 

 inoculation on ordinary agar media. 



Plague.^ 



The bacillus of oriental plague or bubonic pest was discovered 

 independently by Kitasato and Yersin during the epidemic at 

 Hong Kong in 1894. The results of their investigations, which 

 were published almost at the same time, agree in most of the 



1 In revising this subject we have made extensive use of the Report of the Indian 

 Plague Commission, 1901, » 



