Great question! In the spring I added zinc once a week and added tonic to my morning OJ. Maybe they help, maybe not? Summer is easy to eat healthy out of my garden. I had a lapse in cardio for 6 weeks but am back to playing hockey twice a week. It's only pickup games which aren't as rigorous as league games, but it's still good exercise. Adding in time in the sun and some weight work-outs each week and that's pretty much my immune system game.
As well I'm keeping my immune system active by going out amongst the sweaty masses and not wearing a mask in public, so I'm exposed to my normal load of germs and viruses. And when a mask is required, like at the grocers, I do it really half ass and touch my face a lot.
I do worry about all the people secreted away in their homes and their susceptibility to common illness once they come out. With all the concerts, parades and conventions canceled, many people working from home and all the mask wearing, the average person is being subjected to way less illness vectors than in the past, so even a common cold may hit harder than normal this fall and people may be likely to show symptoms they didn't in the past.