My mother is a nurse and has always said the heathiest kids are exposed to other children as well as pets on a regular basis and play outside! Being exposed to the "world" helps to build a good immune system. Unfortunately, for some immunity we need to be exposed to or get sick from a virus. Our body fights off the invader, thus building the immune system.
Although, there are instances where children who have a weak immune system due to certain diseases/illnesses who must stay in a cleaner environment. Most healthy children/people can be exposed to all of the things I mentioned above.