I'm one of those. I got sick A LOT when I was very young. It strengthened my immune system.
I last got flu in the early or mid '70s.
As for colds, there are about 200 rhinoviruses that cause colds. Once you've had one of those and recovered, you are now immune to that specific rhinovirus. Which is why little kids get lots and lots of colds, and older folks not so many.
I am a huge believer in the natural immune system. That does NOT mean I'm opposed to vaccines. Best example is this: If you get smallpox and recover, you will never ever get smallpox again. But chances are you are terribly scarred from smallpox. For a long time any woman who did not have smallpox scars was automatically beautiful. In the context of my appearance, that's sobering. I'm okay looking, and it's weird to think that a couple of hundred years ago I'd be beautiful. However, I'm glad to have lived in an era when we had the smallpox vaccine and for those of you who are younger, live in an era when smallpox simply doesn't exist.
I had what in my childhood were the standard childhood diseases: measles, mumps, rubella, chicken pox. I'm fine with having had them, since I never knew anyone who had any kind of complications from them. That said, I'm aware they can have complications and I'm more than happy to prevent those.
A side note. My son got Fifth Disease, which is the fifth disease after measles, mumps, rubella, and chicken pox, but is no where near as common or well known. It's also extremely benign, fortunately.