Z75 



gilli, which very often cause disease of the lungs, especially in 

 birds. To this branch of science the term parasitology or macro- 

 biology is applied. When the parasitical worms are only studied 

 the science is denominated helminthology. 



The micro-organisms are sometimes spoken of as micro- 

 scopical organisms , or occasionally as invisible organisms. This 

 latter term is only applied when the organism causing disease 

 cannot be seen by the most powerful microscope yet devised. 

 The former, when the microbe can be made out by the means at 

 our disposal. 



Some of the most virulent diseases of animals are now 

 known to belong to the class due to invisible microbes. This class 

 includes the following diseases: rabies, foot-and-mouth disease, 

 cattle-plague, swine-fever, canine distemper, infectious anaemia 

 of the horse, Cape horse-sickness, epitheliosis of birds (commonly 

 termed by poultry and pigeon fanciers "sore-head," chicken-pox, 

 pigeon-pox), leucocythaemia of fowls, blue-tongue of sheep, spot- 

 ted disease of the tobacco plant, bird-plague, cow-pox, sheep-pox 

 and, in man, yellow fever. 



The microscopical microbes cause such diseases as diphtheria 

 of pigeons, poultry and other birds, bird cholera, bird septicaemia, 

 tuberculosis, spirillosis or spirochetosis, etc. 



The common characteristics of the ultravisible viruses are 

 (i) they have not been cultivated in artificial media outside the 

 body ; (2) they resist a high temperature, various chemical agents 

 and even putrefaction ; (3) they do not appear to grow outside 

 the body. 



The mere detection of a microbe in the blood, tissues, or 

 discharges is no proof that it is the specific cause of disease, even 

 though it may be cultivated in a series of artificial culture media 

 outside the body and when injected produces disease. Koch's 

 postulates laid down many years ago are found wanting, and 

 to them must now be added another, viz. : that an animal must 

 be immune to natural contagion before the specificity of a par- 

 ticular virus can be accepted. No doubt as time proceeds many 

 of the alleged specific microbes of contagious diseases of mankind 

 and animals will be proved to be not the specific cause but as 

 secondary organisms only. 



