I read Dr. Klemes story carefully for she described my life. My mother's family are farmers, in my younger years I spent time with them in summer. I believe exposure to the outdoors did improve my immune system. I wound up working outdoors all my life in landscape industry. At my last job about 6 of us had year round employment. I noticed our supervisor was scared to deaf of getting from anything. He had no pets. He was sick all winter every winter for years. The other 5 of us had dogs and cats in the house with us and we never got sick.
Year ago I began to see working outdoors in the soil, dust and dirt didn't kill me. Suffer a minor cut? Lick it off and keep going. Suffer a cut from a greasy piece of machinery? Clean it, wrap you handkerchief around it, decide if you need to go all the way back in to the shop for more advanced first aid. You will recover eventually. I have taken so many drinks from garden hoses with pure, chemical free well water and it hasn't killed me yet.
I have also observed that working outdoors with moderate lever activity has left me in for better health than most seniors my age (71).