healthy means able to digest food, absorb nutrients, and handle foreigners without much immune system involvement.

i keep mine healthy by avoiding preservative chemicals and processed food in general, avoiding antibiotics unless i really need them (tick bite), fasting, eating fermented and rotting food, avoiding stress, eating dirt, swimming in ocean, not going overboard with cleanliness.

most bacterial infections with undesirable results, eg strep throat, resolve for me with a 2-day water fast supplemented with raw onion and garlic and lemon.

in addition to gut, there are biomes everywhere else. i don,t use soap or any other "products" on my body. i only use one soap for all my soapy needs, one made traditionally (bronners solid as of 2020)