680 YELLOW FEVER 



tive to another subject. In summer weather this period is about 

 twelve days ; at a lower temperature somewhat longer. This 

 probably means that, as in the case of malaria, the parasite must 

 pass through certain stages of development before it reaches the 

 salivary gland and is thus in a position to be transferred to a fresh 

 subject. Infected mosquitoes, however, retain the power of 

 infection for a considerable time afterwards, probably as long as 

 sixty days. It has also been shown that mosquitoes may become 

 infective after biting a patient on the first, second, or third day 

 of the disease, but at a later period the results are usually 

 negative, apparently because the virus is no longer present in 

 the blood. 



Interesting results were also obtained with regard to the 

 communication of the disease directly from patient to patient, 

 the conclusion arrived at, after careful experiments, being that 

 the disease cannot be transferred in this way, even when the 

 contact is of a close character. In a specially constructed house 

 seven men were exposed to the most intimate contact with the 

 fomites of yellow fever patients for a period of twenty days each, 

 the soiled garments worn by the patients being in some cases 

 actually slept in by these men ; the result was that not one of 

 those thus exposed contracted the disease. The conclusions on 

 this point have been subsequently confirmed by other workers. 



The American Commission also found it possible to transmit 

 yellow fever to a healthy man by injecting small quantities of 

 blood or of serum taken from a yellow fever patient at any 

 period up till the third day of the disease. The period of 

 incubation in this case is somewhat shorter than when the disease 

 is conveyed by the bite of mosquitoes, the average duration in 

 the former case being about three days, and in the latter about 

 four days, though these times may be considerably exceeded. 

 It is also interesting to know that in these experimental injec- 

 tions the blood or serum used was found to be free from bacteria. 

 Up till the present time, we know of only these two methods of 

 infection, namely, indirectly by the bite of a mosquito infected 

 with the yellow fever germ ; or directly by the injection of some 

 of the blood from a yellow fever patient. In these respects 

 there is a striking similarity to what has been established in the 

 case of malarial fever. 



Experiments with regard to the nature of the yellow fever 

 organism were carried out by Reed and Carroll, and interesting 

 results were obtained. They found that the organism of the 

 disease was very easily killed by heat, as blood from a yellow 

 fever patient lost its infective power on being heated to 55 C. 



