system
Given the following post, respond as if you're commenting on Reddit:
human
Thinking of life-long health, is it better to avoid germs as much as possible or be exposed to them and strengthen your immune system

Will I be healthier if I always wash my hands, always practice food safety and temperatures to a tee, avoided dirt generally, used disinfectants, etc etc, or would it be better if I just dealt with germs, dirt, bacteria, etc (within reason of course).
gpt
Biologist here. Yes and no, as with all things biology. (EDIT: Bonus gamer analogy TL;DR at the end! EDIT 2: clarified a couple of redundant redundancies.)

Immune challenge is one of the ways our immune system learns how to defend against bacteria, viruses and parasites. We have patches of immunological tissue in our gut and other mucous membrane lined tissue in addition to our lymph nodes, thymus, and spleen. It contains multiple types of immunological cells--those that attack and digest potential threats, and those that "present" antigens derived from those now-broken up threats. Antigen-presenting cells can then activate lymphocytes your body has already created. If one of those lymphocytes recognizes the antigen presented, it's not because it has necessarily seen that antigen before--the body is very good at anticipation, and creates receptors with HIGHLY variable paratopes to match epitopes on the presented antigen. Think of the epitope as the key and the paratope on the lymphocyte receptor as the lock. Genes that encode the proteins that build these receptors undergo a process of recombination that creates many cells, each with receptors that will respond to different things. There is a round of selection that only allows immune cells that don't recognize proteins that are supposed to be there (self-antigens) to hang around, i.e., cells with paratopes that recognize self antigens are destroyed. It doesn't want to waste resources on creating things that will harm the body. That still leaves many cells, each with a paratope that will recognize epitopes of a different antigen--locks waiting for their own special keys--called "naive" because they haven't yet been presented with antigen that matches. When a key fits into a lock, the magic begins. Immature B and T lymphocytes undergo a process of maturation in your lymph nodes, and two things happen: the structure and function of the cell changes, and that cell--it only takes one!--undergoes clonal expansion, a fancy way of saying that that cell reproduces itself many, many times exactly, with receptor paratopes matching the antigen epitope that "unlocked" it. (When you get swollen glands, it's partly because of clonal expansion, it happens that fast and that many times.) Immature T lymphocytes become "killer" T cells that can attack precisely and directly, and immature B lymphocytes become plasma cells that become little factories that make antibodies. (Antibodies, by the way, are just versions of the receptor "locks" that are secreted by plasma cells that can attach themselves to the antigen that unlocked the original naive lymphocyte.) Antibodies coat whatever they lock onto, and then activate other parts of the immune system to help out as well as telling other "naive" immune cells to mature. When the initial response is over, a few activated B and T cells hang around in circulation for years afterward, waiting to be unlocked again so that the immune response happens much, much faster next time--this is how vaccines, which are basically just antigens directly injected into the body, work to give lasting immunity. More challenges that match other types of antigen increase both the circulating antibody (more active plasma cells) and circulating T cells.

There's a balance. If your immune system is constantly under challenge, it doesn't work as well because it's too busy responding. Hygiene is important for this reason, and certain viruses specifically overactivate the immune system so it goes haywire and can't actually do its job. But, if you compulsively avoid sick people or keep everything covered in sanitizer, your body never learns to respond to the antigens in your environment by having those circulating mature lymphocytes and antibodies, increasing the likelihood that you'll *have* to respond to too much at once.

TL; DR: It's like unlocking an achievement and getting a special, specific weapon every time an antigen matches a lymphocyte's receptor. Vaccines are like cheat codes. Hygiene makes sure you only encounter what you can handle at one time and respond appropriately, otherwise it's like mashing the jump button over and over until you fall off a cliff. Oversanitizing is like not playing the game at all and then expecting to beat the final boss without knowing what you're doing.