I've always started taking Vitamin C when I get sick, and keep on it for awhile. Eventually stopping because I haven't been sick for awhile and forget about it. Then I would get sick again, and repeat.
Ever since COVID broke out, I've been taking 2,000 milligrams of it every day, and haven't had so much as a a cold. I'm not saying it's a silver bullet, but I've never been sick while maintaining a Vitamin C regimen.
Lack of sleep can be a real huge thing too.
I can't remember the last time I was getting a good amount of sleep and got sick. I'd say 90% of my colds are weeks I pull some all nighters working or traveling. I think the travel is more from lack of good sleep than catching something on the plane because the same thing happens if I skip sleep without leaving the house.