SUSCEPTIBILITY 15 



i.e., much of the virulence is lost. A much larger dose 

 will then be required to produce an effect on a susceptible 

 animal, but the virulence may be enhanced by passage 

 through a suitable animal. 



The virulence of an organism may be ' attenuated ' 

 artificially; for example, by exposing cultures of anthrax 

 to a temperature of 40 C. for some time, they become 

 attenuated to such a degree that they will kill nothing 

 larger than mice. 



While many animals exhibit more or less susceptibility 

 to an organism, some may absolutely resist infection 

 therewith (natural immunity). 



In the case of several diseases, notably in smallpox, to 

 a less degree in measles, mumps, whooping-cough, and 

 scarlet fever, it is not often that the same person is at- 

 tacked twice by the same disease. That is to say, one 

 attack is ' protective,' and in the above-mentioned 

 diseases the ' protection ' may last a lifetime, but extends 

 only to that particular disease, and does not in any way 

 protect against other diseases (acquired immunity}. On 

 the other hand, an attack from certain other diseases may 

 even predispose the patient to a second attack of the same 

 disease. This is true of influenza, diphtheria, pneumonia, 

 and erysipelas. A distinct predisposition may be caused 

 to attack by other diseases thus, diphtheria and scarle.t 

 fever mutually predispose to one another. 



The number of bacteria introduced is important, as a 

 cell or tissue may successfully repel a limited number, 

 but succumb to a greater. 



Combinations of infectious diseases are sometimes met 

 with, such as syphilis with gonorrhoea, diphtheria with 

 scarlet fever, and pneumonia with typhoid fever. These 

 are known as ' mixed infections,' but if, as is generally the 

 case, one disease has lessened the immunity to the one 

 acquired last, the latter is known as a ' secondary infec- 

 tion.' 



Symbiosis. Symbiosis is the cohabitation of two different 

 organisms for mutual benefit, or their co-operation to 

 produce certain reactions. The presence of streptococci 

 appears to enhance the virulence of the diphtheria bacillus. 

 The effect on animals of an organism may be greatly en- 

 hanced by the injection along with it of some other 

 organism that has not pathogenic properties, but which, 



