102 INFECTION AND IMMUNITY. 



at such times. In the first stage of sleeping sick- 

 ness, the so-called trypanosomatic fever, the fever 

 is of a remittent type, and during the attacks of 

 fever the number of trypanosomes in the blood 

 increases. In the stage of sleeping sickness they 

 appear to be limited very largely to the lymphatic 

 glands and the meninges. They can readily be 

 obtained from these sites by puncture, but their 

 presence in the blood is much more inconstant. It 

 has been the experience that on some days pro- 

 longed examination of the blood will disclose no 

 trypanosomes ; on other days two or three per field 

 may be found ; and on still others as many as seven 

 or eight per field. 



cyclic The tertian and quartan infections with the 

 InV Maiariiu malarial parasites, at least in their early stages, 

 illustrate a special type of recurrent generalized 

 infection, which is cyclic in nature, the period of 

 intense general invasion coinciding with a certain 

 stage of the asexual multiplication of the parasites. 

 (See chapter on malaria.) 



(S) Nature and Mechanism of Infection. 



penetration i*y ^ * s to be understood that infection presupposes 

 a penetration of the body surfaces to a greater or 

 less degree. Even in pityriasis versicolor, the 

 most superficial of infections, the fungus pene- 

 trates the horny layer of the skin. Hence, in a 

 consideration of the nature and mechanism of 

 infection, it would be desirable, first of all, to con- 

 sider the manner in which micro-organisms and 

 their poisons may reach the deeper tissues. In 

 some instances this is readily understood, whereas 

 in many others we must in the main be content 

 with mere deductions. 



