It's all about building immunity to different bugs. The cleaner someones home and lifestyle is, then the less likely they are to have built up much resistance to every day germs.
However, I am not suggesting that people forego personal hygiene, washing one's hands after being to the loo, and cleaning one's teeth daily is very important, as is having clean hands before preparing food. But having dust free houses, and immaculte carpets (unless you have a baby who crawls on the floor), is overkill, and potentially more unhealthy than a bit of general dirt.
For people like me, who love having their dog sleeping on/in the bed with them, we probably have better mental health, as well as physical health (not only does having close frequent contact with a dog that you love usually have great benefits where building up immunity is concerned, but it also usually means that the dog and it's walker get much more outdoor exercise than the general populace! Well trained, and established family dogs, definitely have great affects on their human's mental health as well).
May I suggest Satin, that your DD may actually gain some health benefits if she were to roll about in some mud - but not shit obviously - and jump in some muddy puddles, and I maintain that even if she is 32 rather than two years old 😉