And what you arent understanding is that opening the spa, and filling it with unsanitary carbon based lifeforms means it most certainly isnt a closed system.
So, what happens when you go to the beach, lake or pool? Other than the pool with it's unhealthy level of chlorine, there is no protection. So you are at far greater risk than entering my hot tub. You can't live totally isolated from the world. In fact, the pursuit of absolute sanitary conditions makes one more subject to infection because your body does not produce the necessary antibodies to fight infections.
Well at the beach and a lake, Im not sitting int he same 400 gallons of 104 degree ater that me, my family, my neighbors, my neighbors kids have all sweat in, pissed in, **** in ( in micro amounts, but true), ejaculated in, sneezed in, spliied beer in, etc FOR 5 YEARS! At the beach or lake, its trillions and trillions ( beach) and millions and millions ( lake) of water thats not at optimal bactera breeding temp that circulates around and doesnt sit in the same 400 gallon cesspoll that your hot tub is. Is it sanitary? No. is it probabally cleaner/less risky ( and just downright gross) than your hot tub? I wouldnt get in yours to test it out.
I dont get into a lake or the ocean expecting reasonably clean water, nor do I do so often ( less than once a year). Do I expect ultra sanitized and sterile water ina hot tub, hardly, but I do expect you to at least give it some effort.