I just got a virus that has felled all of my co-workers with severe symptoms for a week+. In my case, I felt "kinda sick" for 2 days and then fine. No symptoms beyond that. It was that feeling in the beginning of getting sick where you know you're battling a virus, except in this case I routed the opposing army and slaughtered them as they fled. This was the first such problem in almost exactly 6 months, where I used to get fully sick every 2 months.
I suspect that vitamin D has a lot to do with it, but less inflammation and less available glucose for various pathogens to consume also helps.