A full blood count (FBC), also known as a complete blood count (CBC), does not directly detect viral infections. However, it can provide valuable information that may indicate the presence of an infection, including a viral infection. An FBC measures various components of blood, such as red blood cells, white blood cells, and platelets. In the case of a viral infection, certain changes in the blood cell counts and differentials, such as an increase or decrease in specific types of white blood cells, may suggest an ongoing infection. However, a definitive diagnosis of a viral infection requires specific viral tests, such as PCR or antigen tests, designed to identify the presence of the particular virus causing the infection.