TYPHOID FEVER. 85 



This brings us to the only method in which typhoid 

 fever is now diagnosed by the bacteriologist the 

 Widal's reaction. This reaction is a special example 

 of a general law which was discovered by Durham and 

 others, and which is to the effect that the blood serum of 

 a person who has been through an attack of a bacterial 

 disease will cause the specific organism of that disease 

 to collect into clumps. For instance, if we take a broth 

 culture of the vibrio of Asiatic cholera (which is turbid 

 and opalescent) and add to it a small quantity of blood- 

 serum from a patient who has recovered from an attack 

 of cholera we shall find that the culture becomes clear, 

 a sediment collecting at the bottom of the tube ; and if 

 we examine this sediment we shall find that it consists 

 of felted masses of the vibrios. This reaction is a 

 general one, and is given in most, if not all, bacterial 

 diseases. But Widal, Griinbaum, and others, working 

 independently about the same time, showed that whereas 

 in many diseases it is a reaction of immunity (i.e., does 

 not occur until late in or after the disease) in typhoid 

 fever it is a reaction of infection, and occurs so early in 

 the course of the disease that it is of great value in 

 diagnosis. 



The test is applied by adding a small quantity of the 

 serum from the patient suspected to be suffering from 

 the disease to a larger amount of a young culture of 

 typhoid bacilli, and watching whether the appearance 

 of the culture undergoes any change : it may be 

 watched under the microscope or by the naked eye, the 

 technique differing in the two cases. The microscopic 

 method is rapid and requires a very small amount of 

 blood, and is now generally used. The macroscopic 

 method is perhaps somewhat easier for a beginner, but 

 it takes a longer time and requires a larger amount of 

 blood serum. 



