How does a complete blood count show a possible infection?

Elevated neutrophil: Count. The total white blood cell/leukocyte count (TLC) and especially neutrophil/granulocyte count is usually elevated in patients with bacterial infections. Viral infections and whooping cough are associated with increase in lymphocyte count. Not all infections produce discernible changes in cbc.

Complete: This includes a differential white blood cell count showing the types of cells and their maturity. In bacterial (and some other) infections one may see immature white cells. Infections may also alter the red blood cells, the amount of hemoglobin, etc, and may also change the platelet counts, all of which are measured in a complete blood count.