Do you do anything?

Is there anything you do to try to boost your/your kids immune systems? I’m normally very laid back about this stuff, and believe that we need germs in order to boost immunity, but of course, I also teach hand washing,etc. to prevent the spread of germs. When ds1 gets sick it goes right to his chest. We have a nebulizer and we’ve had to take him to the ER twice due to croup (he was wheezing and just couldn’t catch his breath). Ds2 doesn’t get quite as sick as ds1. Now that school has started, I’m trying to do whatever I can to help ds1 stay healthy. Do you or your kids take vitamins? What, if anything, do you do to to prevent illness?