TYPHUS FEVER. 725 



serum from the immune guinea-pig. The immune 

 defibrinated blood contains antibodies in such con- 

 centration that 0.1 c. c. often protects against 1 

 c. c. of third-day virus, representing anywhere 

 from 30 to 100 minimum pathogenic doses. In 

 other instances 0.3 or 0.4 c. c. of immune blood 

 are required for this degree of protection. When 

 1 c. c. of strong immune blood is injected subcu- 

 taneously into healthy guinea-pigs, the passive 

 immunity is still present in marked degree after 

 twenty days; after thirty-eight days it is present 

 only in such degree that a mild course of spotted 

 fever results when virus is injected; after forty- 

 five days it is no longer manifest. It is possible 

 that passive immunity would not last so long if 

 the immune blood is injected into a foreign 

 species. 



The guinea-pig may be protected against spotted 

 fever following its inoculation with infected blood, 

 provided the immune blood is administered on 

 the second or third day after inoculation. 



The work of Ricketts indicates that efforts at Prophylaxis 

 prophylaxis are to be directed toward the exter- 

 mination of ticks and the wild animals which 

 harbor them. 



Serotherapy will probably depend on the culti- 

 vation of the microbic cause of the disease. 



IV. TYPHUS FEVER. 



Typhus is now a rare disease. It is endemic on occurrence 

 a small scale in London, Glasgow and Liverpool, 

 and cases occur in the larger cities of Ireland. In 

 epidemic form it attacks localities in which the 

 hygienic conditions are bad. The contagion seems 



