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 wmmunity (t.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. 
