114 THE ACUTE SELF-LIMITED INFECTIONS 



serum collected and mixed with the bacterial suspen- 

 sion in dilution of 1 part of the serum to 20, 50, 100 or 

 more parts of the bacterial suspension. These dilutions 

 are used because sera from some persons entirely free from 

 typhoid will clump the bacilli in low dilution, 1 to 5 or 

 1 to 10. The mixture of serum and bacteria is observed 



FIG. 34 



Microscopic field, showing the top of a drop with the typhoid reaction. 

 (Park.) 



under the microscope after they have stood together 

 for a definite time, and the presence of clumping, with 

 loss of movement, noted. In case this occurs typhoid 

 is present. This agglutination reaction is called the 

 Widal test, and is positive in about 95 per cent, of 

 all cases. (See Figs. 33 and 34.) 



Cultures. It is also of aid in the diagnosis of typhoid 

 to make a blood culture. This consists in withdrawal, 

 under sterile conditions, of blood from a vein, placing 



