What is CBC test by getting blood sample, that?

comes under HAEMOTOLOGY and result says normal value 4,000 to 11,000 cells/cumm and observied is 12,200 cells /cumm The Doctor says there is a infection and pric some antibiotic. my ques is what causes this infection, this is in blood or stomach , impact on diaditic pat , is there is chance of inf geeting out through urien or stools. thgs