How would agglutination reactions be used to locate the source of an epidemic?

The agglutination reactions clinically exhibit a broad spectrum of applications. They facilitate in finding the existence of antibodies or antigens in the sample. The agglutination reactions can be used to find the epidemic source. This can be achieved by testing the blood sample of the patient.  

If the antibodies found in the sample of blood possess the tendency of undergoing agglutination with the disease-causing antigens, then it can be considered that the person has got encountered with the disease. In case of an epidemic, the samples of blood from the group of individuals is taken and examined for the agglutination reaction. Thus, the reaction can be used to find the source of the epidemic.  

idk what the question is about