Infant and toddler HBV carriers are primarily infected through horizontal transmission, which can occur due to blood transfusions, vaccinations, or close contact with HBV-infected family members. Young children under 3 years old are also prone to chronic HBV carriage due to their underdeveloped immune systems. Asymptomatic HBV carriers refer to individuals who test positive for hepatitis B surface antigen but show no liver inflammation symptoms and have normal liver function tests, with no changes observed over half a year.