It's all about building up your immune system, vaccination are a small dose of the virus and once the body deals with it, it can deal with future contact with viruses.
After the Berlin wall came down they found that children of East Germany got sick less often than children of West Germany, it was put down to the fact that children of West Germany grew up in a more sanitised environment where children of East Germany grew up in rather un-sanitised environment and played in the dirt.
