I literally haven't been sick in 9 years, coinciding to when I started taking 5-10,000 IU of Vitamin D/day. Just this past weekend, I spent 5 entire days inside a small camping trailer (during the sleeping hours) with a buddy of mine who was sick as shjt with some type of Rhinovirus (no fever). I caught it, but it was gone in like a day and a half. Who knows if/what actually boosts immunity, but I take (things that have been linked to immunity): Swanson Coagulans, Plantarum, and Reuteri pro-biotics, I eat a cup of Oikos Triple Zero Yogurt per day, Sauerkraut, ACV, 10 gelcaps (5/5) of some Walmart (Nature Made, Nature Valley?) Odorless Garlic, and Swanson's Sub-lingual Glutathione. I also eat 99.8% "Clean" and don't drink or smoke. Who knows, give some of those a shot.