100 INFECTION AND IMMUNITY. 



reach the circulation, persist in that situation until 

 the infection terminates either in death or recovery, 

 in the latter case being exterminated by the pro- 

 tective agencies of the blood. T}^phoid and para- 

 typhoid fever, plague, Eocky Mountain spotted 

 fever, Malta fever, and probably the acute exanthe- 

 mata, are of this type. Eecovery and the steriliza- 

 tion of the blood, however, does not mean that the 

 whole body is necessarily rid of the micro-organ- 

 isms; the latter still may persist for a greater or 

 less period on one or more of the body surfaces, as 

 in the case of typhoid fever, in which the bacilli 

 may persist in the intestines or the bladder for a 

 long period, or in plague in which the organisms 

 may be found in the sputum for some time. 

 Through some accident typhoid may vary from its 

 usual habit, a point which is illustrated by the 

 occasional occurrence of relapses, or by the locali- 

 zation of the bacilli in some solid organ of the 

 body, as in the vertebrae or the muscles, resulting 

 in post-typhoidal complications. 



Periodic ^ n other instances the systemic invasion is of 

 infection^ periodic character, and of this there are a number 

 of varieties. The streptococcus, when it exists as 

 the cause of fibrinous endocarditis or of thrombo- 

 phlebitis, often invades the circulation in a fluctu- 

 ating manner. Periods when there are very few 

 cocci in the blood will be followed by others in 

 which they are very numerous, and the clinical 

 symptoms usually correspond with these fluctua- 

 tions. A similar course probably is followed by 

 tuberculosis and blastomycosis in their invasion of 

 the blood. Mechanical factors sometimes precipi- 

 tate a systemic distribution, such as the rupture of 

 a caseous nodule into a vessel or lymph channel in 



