302 BACTERIOLOGY FOR NURSES 



They are, however, only moderately resistant to heat ; exposure 

 to 45 C. to 50 C. for half an hour destroys them. 



Experimentally the disease may be induced in monkeys by 

 intracranial inoculation or by rubbing the virus on the sound nasal 

 membrane. In such cases the incubation period averages from 

 eight to nine days, but may range from five to forty days. Mon- 

 keys which have recovered from an attack of the disease are im- 

 mune to fresh inoculations ; also it has been shown that the serum 

 of such animals when mixed with infective material has the power of 

 neutralizing the virus to a certain extent. Furthermore, the serum 

 of recently recovered human cases when injected into new cases 

 within the first forty-eight hours is capable of arresting paralysis. 



Since the worst epidemics occur in summer and in rural rather 

 than urban communities the view has been advanced that an 

 insect may be responsible for its transmission. Experiments have 

 shown that a blood-sucking fly when infected may transmit the 

 virus to monkeys. It is doubtful, however, if the fly is responsible 

 for the spread of the disease ; there is more evidence at the present 

 time that its dissemination is due to contact, direct or indirect, 

 with the virus discharged in the secretions from mouth or nose. 

 The existence of healthy carriers may perhaps explain the outbreak 

 of the various epidemics. 



Yellow Fever. The disease is an acute infection occurring 

 chiefly in tropical countries and characterized by fever, jaundice, 

 and hemorrhages. Several investigations have been made con- 

 cerning its etiology, the most extensive probably being that of the 

 United States Commission. The members of the Commission, 

 Reed, Carroll, Agramonte, and Lazear, began their work in 1901, 

 and although they did not succeed in demonstrating the causal 

 agent they discovered facts concerning the transmission of the 

 disease that has led practically to its eradication in areas where 

 the necessary precautions have been taken. 



In Havana preventive measures were first enforced in 1901, 

 and within ninety days the town was free of yellow fever. Several 

 weeks later new cases appeared, but the same measures were ap- 

 plied and the disease was quickly stamped out. 



