PARASITES 431 



positive protection against an attack." Obviously, then, 

 everyone should be vaccinated in infancy and at least once 

 after reachin'g maturity. Anyone who travels or mingles 

 much with people in the winter time should be vaccinated 

 every four or five years. 



DIPHTHERIA 



There is probably no other serious bacterial disease which 

 attacks the human race whose story is so well known as is that 

 of diphtheria. Our knowledge of the details of this disease 

 and of its cure and prevention is well-nigh perfect and it 

 thus constitutes the most complete triumph of the science of 

 bacteriology. This being the case, a somewhat detailed 

 account of the disease will serve to acquaint us with the general 

 theory of bacterial diseases, their cure and prevention. 



499. Discovery of the Organism and Its Relation to the 

 Disease (Bacillus diphtheriae) . In 1883, KLEBS discovered 

 and described the organism and in the following year LOFFLER 

 succeeded in obtaining pure cultures of it from the throats of 

 patients suffering with the disease. This seemed to indicate 

 plainly that the organism is the specific cause of the disease, but 

 Loftier himself was at that time inclined to doubt this, for 

 he had found the same organism in the throats of perfectly 

 healthy children and at the same time had failed to find it in 

 the throats of some patients who showed strong symptoms 

 of the disease. These causes for doubt have since been com- 

 pletely cleared away, however, for it is now known that cer- 

 tain other bacteria can cause a condition of the throat which 

 is practically indistinguishable from that caused by the 

 diphtheria bacillus, and again it is now known that some people 

 carry diphtheria bacilli about in their mouths and throats and 

 yet are entirely unaffected by them. This latter fact is an im- 

 portant factor in the spread of the disease, for people who thus 

 carry the organism about may give the disease to more sus- 

 ceptible persons. 



The final discovery which proved beyond all doubt the 



