Good sanitation is fine and required in institution's like hospitals, however if one does not allow the immune system to deal with threats itself first, then it may not learn to deal with certain things at all, so although you may get less illnesses per year, any time you do get them, frankly will probably be aggravated by the fact that your system is "Lazy", it also hinders evolution somewhat, but that is beyond the scope here.
Drugs also bring in its own problems, we only enforce an environment for the viruses and bacteria to evolve to ignore the drugs effects, but in that process may get more deadly as well.
Being clean is good, being paranoid is bad, it is a false sense of security for one, which if the paranoia hasn't ruined the mental state of that person already, the diminishing returns definitely will.
I dont know really what to say about cleanliness, its really up to the person who wants to clean, however drugs need to advance somewhat, which seems to be coming along, if they cannot solve it then we can always replace the immune system with nanobots, which could also potentially be used to cure cancer (not cure but, deal with extremely well)...if of course they can find a way past the toxicity of Nanometre level objects.
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