A complete blood test to distinguish a viral infection is usually done by looking for a rise in lymphocytes, while white blood cell levels are often normal, except in special cases such as cytomegalovirus infection. A complete blood test is performed to see if a viral infection is present, mainly by looking for a definite rise in lymphocytes, while white blood cells are usually normal or slightly elevated. The presence of heterogeneous lymphocytes is more supportive of a viral infection. In addition, a rise in lymphocytes with a rise in white blood cells can occur with specific viral infections, such as cytomegalovirus infection. However, a full blood test is not the only diagnosis of viral infection, but can be combined with the results of imaging and pathology to clarify the diagnosis. Therefore, it is recommended to consult a doctor for timely examination and treatment.