Hepatitis B virus spreads from one person to another through contaminated body fluids or blood. It is never spread by air, food, or water. A baby can be infected at birth if the mother is infected. Infection can also spread from one child to another through close contact and from one adult to another through sexual activity or the use of contaminated needles. Other possible ways include medical treatment in countries where contaminated needles or blood may be used. Preventing the spread of the virus from mother to child and from child to child is particularly important because chronic hepatitis B infection (with the increased risk of liver cancer) is much more common after infection in the first 5–10 years of life.