I've found that taking the following really helps me not to get sick: 10,000-20,000 IU Vitamin D3 (Works best if you also take Vitamin K2 and Magnesium. The K2 puts calcium in your bones and teeth instead of letting D3 put it in your soft tissue. Magnesium activates the D3. All 3 work with Calcium, which I take as well). If you are getting lots of sunlight you might make do with 5,000 IU, but most people, especially in the winter, get very little sunlight and therefore don't make Vitamin D. If your levels are low, a loading dose of 30,000 IU for a week can quickly raise your levels. Most doctors aim for blood levels of 30-80 ng/ml, but that is because of a misunderstanding. When testing for overdoses, it was found that a person could overedose at 300 ng/ml (and that was without K2, so probably could go higher with K2). The researcher then arbitrarily recommended <100 ng/ml. Doctors then saw 100 ng/ml as the danger level and started recommending 30 ng/ml (barely above level for ricketts). Natural doctors have recently raised their recommendations to 50-80 ng/ml, but that is probably still low. I recommend blood levels of 80-120 ng/ml, but I've gotten just over 150 ng/ml and that was the most illness resistant I've been. If you regularly take more than 10,000 IU, it is a good idea to get a blood test because different people absorb it differently. 15-30 mg of Zinc (You need Quercetin or another Zinc ionophore to get it into your cells. Also, high zinc can hurt copper metabolism, so taking a little copper is good. I take Zinc Balance that has both in the right proportions.) 1500-3000 mg of Vitamin C split into 3-4 doses throughout the day because it is water soluble and doesn't stick around too long. 500 mg 3x/day is pretty good. Optimally, increase doses until you get loose stools and then reduce by 500 mg. You may need more than usual when you are sick. Either using a neti pot nightly or using Xlear nasal spray 1-2x/day can also clear out viruses from your nasal passages before they have time to get established. Daily is best, but using one of them the moment you get a hint of sniffle or tickle in the back of the throat can work, if you truly use them the moment you might have a hint of something. It is less effective if you wait until you are actually sick. This routine has helped me to almost never get sick. I started in 2020 because of Covid and continued because it stopped everything else as well.