@itiwhetu so I know this is a bit off topic, but this reminds me of a study I read a couple years ago about why kids get more sick these days than they use to.
Whole group of medical professionals from around the country got together to collaborate on the study (I've been looking for like 10 min. before typing this and can't find it yet) and they concluded that: kids get sick more easily because they don't eat enough dirt from playing outside like older generations did as children. Since they're not exposed to as much, they don't build up the immune system that they could have.
Like I said it's off topic, and I don't really agree/disagree with anything in this thread since I'm not as knowledgeable on this topic (I'll research more later, it's on my long, long, list of items to study now), but thought you might find that interesting.
If I find the study I'll be sure to post, it was intriguing.
If you ever come across any cited or peer reviewed info. about this be sure to post! I'd be interested in checking it out too.