One very important thing to strengthen our immune systems is to avoid ALL processed foods. I cook from scratch for the most part, don't buy anything that has a long list of ingredients, many of them various chemicals. I've done this forever. We also almost never eat out. My daughter never had colds/flu...until she moved out at 24, started eating packaged foods, and had so many respiratory infections that her insurance dumped her. All that was 26 years ago.
My husband traveled, doing nine trade shows in the outdoor industry every winter, some of them back-to-back. Days of driving, followed by other exhibitors and the public all hacking, sneezing, coughing. He's a brittle diabetic and, during those years, had very low kidney function. I pre-cooked meals for him to heat up in the microwave in his room; he made sandwiches and took fresh fruit for lunches. In spite of his health issues, he never, ever had even a sniffle all those years! He quit doing shows for the company when he went on dialysis, subsequently got a kidney transplant six years ago and, of course, is on immunosuppresents. He's never had a cold or flu since the transplant. We quit doing anything other than simple hand washing 5½ years ago.
Avoiding the additives and chemicals in processed foods is a huge step towards staying safe. Cooking many things from scratch is not terribly difficult. I do use a few canned items, like tomato paste (not sauce, which is full of salt) that have nothing other than the food itself. Naturally, I avoid any canned item that has salt. Due to his diabetes and both of our kidney issues, I avoid using salt, sugar, and fat in everything I prepare. Making pasta sauce from canned tomato paste only requires adding herbs and enough water to yield the right consistency, with a pinch of sugar to bring out the natural tomato flavor. How much more difficult is that than heating bottled sauce that's full of chems and salt, perhaps sugar?