A routine blood test is usually required when a child has a fever. This test is used to determine if there is an infection and the type of infection. In viral infections, it mostly shows a reduced white blood cell count in the peripheral blood; in bacterial infections, it suggests an increased percentage of white blood cells and neutrophils; in viral infections, there are abnormal lymphocytes in the peripheral blood; and if it is caused by leukaemia, infantile cells are usually found.