I think it should be stated that areas with lack of biodiversity are associated with increases in autoimmune and mental disorders, we do not know that they cause these increases. That said, it is not a big leap to see our health deteriorate as we drift further from our evolutionary niche. Nature is a symphony of effects that together support life as it was evolved to be. If we disturb this symphony it is reasonable to assume there will be problems. As the author notes, it does not take large efforts to return to some of our natural environment.