314 THE STREPTOCOCCUS-PNEUMOCOCCUS GROUP 
current bacteriological methods. Tendencies toward slow changes 
may be used as further valuable distinguishing characters."^ 
Scarlet Fever.— Hemolytic streptococci have long been suspected 
of being etiological factors in scarlet fever. They are almost invariably 
found in the throats of scarlet fever cases. They are obtained 
not infrequently in blood cultures made during life, and they are 
isolated quite commonly from the tissues at autopsy. Also, and 
this is significant, a rash sometimes develops about wounds that are 
infected with streptococci, and in an occasional case of puerperal fever. 
Attempts to reproduce the rash in experimental animals have always 
been negative, a fact which seemed to discredit the etiological relation- 
ship of streptococci to the morbid process. It is now known that 
animals are insusceptible to the rash-producing constituent of the 
scarlet fever poison. The first real demonstration of the etiology of 
scarlet fever was by Drs. George and Gladys Dick.- They obtained 
a culture of streptococcus from a case of scarlet fever and inoculated 
some of this culture into each of several persons. One developed a 
mild, but typical case of the disease. Some of the filtrate of the culture 
was injected into other persons. It was found to be ineffective in caus- 
ing the disease. This seemed to eliminate the possibility of a filterable 
virus. Finally some of these same persons that had received the 
filtrate were inoculated with the culture from which the filtrate was 
made and one of them developed a typical attack of scarlet fever. In 
this manner was the etiological relationship of certain strains of hemo- 
lytic streptococci to scarlet fever determined. With this specific 
knowledge of the etiological relationship of streptococci to scarlet 
fever clearly in mind, it is possible to look back through the voluminous 
literature and realize that a very few previous observers may have 
been quite correct in their surmises of the etiology of the disease.'' 
The Dicks also discovered that their scarlet fever streptococci pro- 
duced a soluble poison, which when filtered from the organisms, was 
found to be relatively heat stabile. With this poison suitably diluted, 
they tested the susceptibility of a considerable number of persons to it, 
and thus presumably to scarlet fever, precisely as persons are tested for 
their susceptibility to the soluble toxin of the diphtheria bacillus, and 
therefore to the disease diphtheria. The Dick test is to scarlet fever 
what the Schick test is to diphtheria. 
In order to perform the Dick test properly, it is necessary to have a 
standard preparation, which shall contain the least amount of the toxic 
filtrate of a suitable streptococcus culture that will produce a reaction 
when it is introduced intradermally in a frankly susceptible person. 
This is known as the Dick skin test dose. In such a person a reddened 
area, about 2 cm. in diameter forms about the site of injection in about 
' Smith and Brown: Jour. Med. Res., 1914, 31, 501. 
2 Jour. Am. Med. Assn., 1923, 81, 1166. 
' An interesting summary of some of these early investigations is by Park, Jour. Am. 
Med. Assn., 1925, 85, IISO. 
