1891.] on Infectious Diseases. 283 



While, therefore, we know in these cases on what the conditions of 

 infection depend, we have also learned to understand the conditions 

 which favour or prevent the infection. 



Not all infectious diseases which have been studied are due to 

 Bacteria : in some the microbe has not been discovered, e. g. hydro- 

 phobia, small-pox, yellow fever, typhus fever, measles, whooping- 

 cough ; in others it has been shown that the disease is due to a 

 microbe which belongs, not to the Bacteria, but to the group of those 

 simplest animal organisms known as Protozoa. Dysentery and 

 tropical abscess of the liver are due to Amcebse ; intermittent fever or 

 ague is due to a protozoon called fl?emoplas medium : a chronic infec- 

 tious disease prevalent amongst rodents, and characterised by deposits 

 in the intestine, liver, and muscular tissue, is due to certain forms 

 known as Coccidia, or Psorospermia, A chronic infectious disease in 

 cattle and man known as actinomycosis is due to a fungus, the 

 morphology of which indicates that it probably belongs to the higher 

 fungi ; certain species of moulds (e. g. certain species of Aspergillus 

 and Mucor) are also known to be capable of producing definite infec- 

 tious chronic disorders ; and so also is thrush of the tongue of infants ; 

 ringworm and certain other diseases of the hair and skin are known 

 to be due to microbes allied to the higher funo-i. 



The microbes causing disease which have been studied best, are 

 those belonging to the groups of Bacteria or Schizomycetes or fission 

 fungi (they multiply by simple division or fission) ; most species of 

 these have been cultivated in pure cultures, and the new crops have 

 been utilized for further experiments on animals under conditions 

 variable at the will of the experimenter. 



[3. Demonstration : cultures of Bacteria in plates and in tnbes.l 



One of the earliest and raost important discoveries was that made 

 by Pasteur as to the possibility of attenuating in action an otherwise 

 virulent microbe — that is to say, he succeeded in so growing the 

 microbes, that when introduced into a suitable animal they caused 

 only a mild and transitory illness, which attack, though "^mild, is 

 nevertheless capable of making this animal resist a second virulent 

 attack. Jenner, by inoculating vaccine, inoculated a mil.l or attenuated 

 small-pox, and by so doing protected the individual against a virulent 

 small-pox. Pasteur succeeded in producing such an attenuated virus 

 for two infectious diseases — chicken cholera and splenic apoplexy or 

 anthrax; later on also for a third — swine erysipelas. For the first 

 two he produced cultures grown under certain unfavourable con- 

 ditions, which owing to these conditions lose their virulence, and 

 when inoeuluted fail to produce the fatal disease, which they would 

 produce if they were grown uu'ier normal conditions. What they 

 produce is a transitory mild attack of the disease, but sufficient to 

 protect the animal against a virulent form ; thus in anthrax he showed 

 that by growing the Bacillus anthracis at a temperature of 4:2 - o to 

 43^ C. for one week, the bacilli become slightly weakened in action ; 

 growing them for a fortnight at that temperature, they become still 



