DISEASES OF UNKNOWN ETIOLOGY 311 



weeks during the period for which its potency is guaranteed, and 

 if one of these tests fail the vaccine is called in. 



The immunity produced by successful vaccination is of rela- 

 tively long duration, it may last from ten to fifteen years. Never- 

 theless it is well, when liable to exposure, to revaccinate at the end 

 of a year. 



DISEASES OF UNKNOWN ETIOLOGY 



Measles. Cell inclusions, bacilli, and cocci have all been de- 

 scribed by different investigators as possibly associated with the 

 etiology of measles . The reports, however, have not been confirmed 

 and as yet the causal agent of the disease is unknown. Hektoen 

 in 1905 succeeded in experimentally producing measles in two medi- 

 cal students by the subcutaneous injection of blood taken from a 

 measles patient during an early stage of the disease. Anderson 

 and Goldberger in 1911 reported having produced the disease in 

 monkeys by means of a filtrate of measles blood. All efforts to 

 cultivate the virus have failed. 



Scarlet Fever. The causal agent of the disease is still unknown. 

 Streptococci have been repeatedly found in large numbers in the 

 throats of scarlet fever patients and for this reason have been 

 considered by certain authorities as the possible inciters of the 

 disease. Other workers regard them merely as secondary invaders. 



Mumps. Although an infectious disease mumps has been 

 little studied ; serum taken from recovered cases has been shown 

 to contain protective bodies. The organism giving rise to the 

 disease, however, has not yet been demonstrated. 



Rocky Mountain Spotted Fever. The disease is characterized 

 by fever and an hemorrhagic eruption. Diplococcoid bodies have 

 been described as present in the blood of infected patients. Similar 

 bodies have also been found in the glands of ticks who have fed 

 upon such patients. Numbers of these supposed organisms may 

 also be found in the larvae of infected female ticks; but their 

 causal relationship to the disease is not proved. 



Chickenpox. No specific organism has been demonstrated in 

 connection with the disease. It has been claimed that a degree 

 of immunity may be conferred by vaccination with the clear con- 

 tents of the vesicles. 



