CH. VI] VARIATIONS IN VIRULENCE 73 



outbreak — for example on board a ship or in a military 

 camp — but in a widespread epidemic — in a crowded city, for 

 instance, — both weak and strong individuals are exposed 

 equally to infection as the disease extends. 



2. Again, the same disease may either take the form of 

 an epidemic or ocGur sporadically in the form of isolated 

 cases, apparently unconnected with each other. In speaking 

 of meningococcal meningitis, Koptik attributes the diminished 

 infectivity of the sporadic types to the senility and weakened 

 virulence of the organism concerned. 



3. A similar diminution in virulence is observed in the 

 case of some endemic diseases in the course, of many genera- 

 tions. Bahr (1912) quotes evidence to show that in Fiji, 

 dysentery 25 years ago was a much more virulent disease 

 than it is a,t the present time. The virulence of the specific 

 virus of syphilis has been modified considerably in those parts 

 of Europe in which it has been prevalent for centuries, such 

 as Spain. 



Here again the development, by those exposed to infection, 

 of increased powers of resistance or "immunity," no doubt 

 plays a part, for when the infection is introduced from places 

 where it has long been prevalent to places where previously 

 it has never been met with, the disease may from the first 

 assume a virulent type. 



4. Epidemics of a particular disease vary in virulence at 

 different times and at the same time but in different places. 

 Typhoid fever is, as a rule, much less severe in England than 

 the same disease in the tropics or in the temperate regions 

 of South America, although the organisms, apart from the 

 question of virulence, appear to be identical. 



5. Again a pa/rticvlar species of organism may produce, 

 at different times, diseased conditions widely difiering in 

 their intensity. The classical example of this is the strepto- 

 coccus pyogenes which may produce at one time merely a 

 local suppuration, at another a spreading erysipelas and at 

 another a rapidly fatal septicaemia. 



6. Many pathogenic organisms when grown outside the 

 body, under various abnormal conditions, lose their virulence. 



