chobani claims six strains of yogurts, I used that as starter for my homemade yogurt batches since then. Otherwise I try and get my exposure in from the environment. I'm vaccinated so I don't bother with the mask unless there are hard rules. I will avoid overusing hand sanitizer. I take crowded public transit and otherwise walk around sidewalks and stores with a bunch of people vs private car and delivery of all my needs. I do computer work but I do almost all of it outdoors on a patio table, where I am exposed to pollen and spores and microbe aplenty (but probably better air quality than indoors given the plastic off gassing in the modern home). Basically I am trying to inoculate myself with a wide variety of things available in my local environment, just like people used to be before all this modern society stuff locked us sitting in rooms. It seems to work as far as I can tell anecdotally; I can't remember the last time I was sick.