Risk Factors for HBV Infection Although relatively rare in the United States, is endemic in parts of Asia where hundreds of millions of individuals may be infected. HBV is transmitted horizontally by blood and blood products and sexual transmission. It is also transmitted vertically from mother to infant in the perinatal period which is a major mode of transmission in regions where is endemic. The blood supply in developed countries has been screened for HBV for many years and at present transmission by blood transfusion is extremely rare. M ....