Question

why are majority of children in many parts of india already immune to hepatitis although they have never been vaccinated against it

Answer 1

They have come into contact with mild concentrations of the pathogen through stagnant water/soil.This has made the immune system to produce antibodies in response to the disease.Thus,the children are already immune even though they may not have been vaccinated.

Hope that helped.
^_^

Answer 2

mild form of this virus is present in water acts kinda like a vaccine