“The hygiene hypothesis” is some new scientific thinking that is getting a lot of serious attention.
Basically it states that your body evolved in an environment where it expects to be under constant attack from microbes and parasites. If you remove all these threats it has side effects. Specifically, your body “knows” that their are parasites and microbes attacking you constantly. If it can’t find them, it attacks the the parts of itself where it expects the invaders to be, or it attacks the parts of itself that remind it most of these invaders. There is evidence that several allergic reactions and diseases like diverticulitis are actually your body attacking itself in the absence of these invaders. Infecting yourself with intestinal worms seems to lessen the symptoms of the latter.
Encountering pathogens an letting your body deal with them seems to strengthen its ability to do so in the future.
Rather than doing anything useful in the long term, products like Purell may be mainly breeding disinfectant resistant bacteria.
I think this makes a lot of sense, but I don’t get crazy about it. Hygiene is extraordinarily important in hospital settings and in food prep. Personally, I don’t wash my hands unless someone is watching. I never use Purell or soap during the day, or anything like that, I put my hands in my mouth and touch my face and grab door handles, and get dog kisses and mess around in the dirt.
My wife uses purell, by the gallon and is a real germaphobe. She gets sick a couple of times a year. I think I’ve had one cold that was worth mentioning in the last 5 or 6 years. I suspect though that this has more to do with the fact that I run and work out every day and eat really healthy, than hygiene.