I don't quite subscribe to the philosophy that says you have to exposed to all the viruses at one time or another and that you will get sicker children at playschool/school if they don't go to nursery.
Viruses mutate from year to year, that's why there's a different flu jab every year. Having got it one year doesn't mean you won't get it the next, and certainly in five years time at school age this is irrelevant.
Secondly, many of the things like conjunctivitis or D and V and so on are contagious, so if you are not in contact with people with it- you won't get it. You don't need to get these things to provide immunity and you can also get them repeatedly if repeatedly exposed. Some D and V viruses change every 6 weeks, so even if you've had it once, you can get it again in the same season.
I have had friends with children in nursery as babies and it has been a misery of illness and stress, with repeated D and V, conjunctivitis, colds that end up as ear infections and so on.
I know part of it is luck but perhaps being repeatedly exposed to infections isn't a good idea if you are already ill through lowered immunity. They seemed to be in a permanent round of antibiotics.
I know this isn't a popular view on MN, and of course if your children don't get sick much, nursery is a really good option because it doesn't matter if the staff go off sick, they are still open. I still think you are more likely to get sick if you are around sickly babies all day and would factor this in to my decision-making if I had a baby that got everything going- they can end up being pretty much ill the whole of their first year.
I did used to avoid baby groups if my dd had had a bad couple of weeks of illness though, she would go and within two days, be ill again. There got to a point that the fun of going to them was taken away by knowing it was just a huge germ exchange, I think partly exacerbated by the fact that the toys were never washed and the whole place a bit manky. Sometimes I just couldn't face another sleepless night over an avoidable illness and surely some are avoidable.