Probiotics and iodine. Everything else like exercise and vitamin C seems obvious in theory even if people forget, but probiotics and iodine aren't obvious. For probiotics I would just eat some Greek yogurt, non-Greek yogurt, sauerkraut, kimchi, or something with Ganeden BC30 (bacillus coagulans is probably the main probiotic you need to consume if you've had digestive issues or used antibiotics ever, so get the best version,) and for iodine I'd just use iodized salt if you don't live somewhere with a lot of available seafood.