



BACTERIA IN DISEASE. 169 



In laboratory work the virulence is usually maintained 

 best by inoculating the bacteria from time to time into sus- 

 ceptible animals. Bacteria coming freshly from infected 

 animals are likely to be highly virulent. The virulence 

 may be increased by beginning with an especially sensitive 

 animal like a very young guinea-pig, and progressively in- 

 oculating into less sensitive animals. The infection of rela- 

 tively insusceptible animals may sometimes be produced by 

 the injection of a very large dose of the bacteria. The 

 addition of the toxic products of the bacteria, which may 

 be obtained by using large doses of cultures in bouillon, 

 makes infection more likely. Cultivation on a particular 

 medium may maintain or increase the virulence. 



Finally, the combination of two or more kinds of bac- 

 teria may produce infection when neither one would do so 

 alone. On the other hand, it is said that the fatal effects 

 of an inoculation of virulent anthrax bacilli into a sus- 

 ceptible animal may be averted if the animal be inoculated 

 with a culture of bacillus pyocyaneus shortly afterward. 



Mixed Infection. It is not uncommon in disease to find 

 two kinds of bacteria associated together, producing a 

 mixed infection. In diphtheria, very frequently, the ba- 

 cillus of diphtheria is found to be accompanied in the mem- 

 brane by the streptococcus pyogenes. The course of the 

 diphtheria may be modified in this manner. The term 

 secondary infection is rather loosely used. It is sometimes 

 employed to designate an infection occurring in an indi- 

 vidual, the resisting power of whose tissues has been weak- 

 ened by some chronic organic disease. Such an infection 

 is often called a terminal infection. Terminal infections 

 are very common in cases of carcinoma, chronic nephritis, 

 arteriosclerosis, and in many other diseases. 



Concerning terminal infections Osier says : " It may 

 seem paradoxical, but there is truth in the statement that 



