Infection with Hepatitis B virus (HBV) at bih or a very young age is associated with chronic HBV infection and the development of hepatocellular carcinoma later in life. In fact, infants born to Hepatitis B surface antigen (HBsAg)-positive mothers are commonly infected, and approximately 90% become chronic carriers of the virus. Chronic carriers suffer from hepatocellular carcinoma at an incidence over 200 times higher than a noncarrier. The current recommendation for infants born of HBsAg-positive mothers is administration of hepatitis B immunoglobulin (HBIg) in the delivery room, with the first dose of the hepatitis B vaccine given at the same time or within 1 week. The second and third dose of the vaccine are then given at 1 and 6 months. With this protocol, 94% protection is achieved. The Epstein-Barr virus is the agent of heterophile-positive infectious mononucleosis. In children, primary EBV infection is often asymptomatic. The measles virus often causes a more severe disease in adults. Over the age of 20, the incidence of complications, including pneumonia, bacterial superinfection of the respiratory tract, bronchospasm and hepatitis, is much higher than in children. Poliovirus causes asymptomatic or inapparent infections 95% of the time. Frank paralysis occurs in approximately 0.1% of all poliovirus infections. However, the probability of paralysis increases with increasing age.