The thing that really a noticeable difference in my immune system was working in a bar. You come into contact with so many people, many of whom don't take good care of themselves, and end up collecting / washing the glasses they have been drinking out of, shaking their hands, handling their IDs, wrestling them out of the building. I was sick constantly for like six months, but after that, I almost never got sick again.

Sadly, it appears to be wearing off. I was sick for a fair bit last winter, and it'll probably be the same again this year.

Purely anecdotal, but I eat lots of unpasturized fermented food, I re-use silverware/dishes/cutting board with just a quick moist cloth wipe, and I haven't been sick in around 7 years. Occasionally I'll notice increased mucus and mild inflammation my nose/throat that tells me I've been exposed to something, but it always goes away within about 8 hours.

I noticed this as well when I started taking probiotics, eating unpasteurized cheeses, and having sauerkraut or kimchi every other day or so.

I'll get a sniffle for a day and then that'I'll be the end of it. I haven't been sick in a year.