218 DISEASES OF UNKNOWN ETIOLOGY 



insect is only capable of depositing it in a bite when 

 twelve days shall have elapsed since it bit a yellow fever 

 patient. More than that, five days elapses between the 

 bite of the mosquito and the appearance of the virus in 

 the patient's blood. Furthermore, the patient's blood 

 only remains infective for three days. The virus is 

 so small that it will pass through the unglazed porce- 

 lain filters used in the laboratory. Because of these 

 facts the modern conception of yellow fever supposes 

 a protozoon as the cause. There are no laboratory 

 diagnostic measures nor as yet any specific treatment. 

 The spread of yellow fever is prevented by destroying 

 the breeding places of the mosquito, a difficult thing, 

 since this insect breeds in lowlands and bushes and in 

 houses. It bites usually in the late afternoon. 



Scarlet Fever and Measles. These two easily com- 

 municable diseases are transmitted apparently by the 

 virus cast off in scales from the skin. Nevertheless 

 the specific causative agent is unknown. It has lately 

 been shown that measles is transmissible to monkeys 

 at some particular time of the disease, which appears 

 from present knowledge to be quite early. The most 

 recent researches fail to show that measles scurf will 

 carry the disease to monkeys. Streptococci were once 

 thought to be the cause of scarlet fever. It would 

 seem from results obtained within the last year that 

 these organisms may be exonerated. 



Typhus Fever. Although this condition is not under- 

 stood clearly, it now seems that body lice, flies, and 

 ticks transmit it. 



Poliomyelitis. This is an acute apparently infectious 

 disease characterized by a mild constitutional illness 



