Both echinacea and zinc do terrible things to my digestive system. I'm also vaguely under the impression that one or both of them don't mix well with allergies - where the problem isn't so much a "bad" immune system so much that the immune system is busy fighting the wrong things. I already take a good multivitamin, and extended-release vitamin C, combined with a pretty good diet containing multiple fruits & vegetables and industrial quantities of innocent smoothies. I've started washing my hands religiously when I get in from "the outside", even if I don't think I've touched anything. Beyond that... um.
The only sensible thing I can think of is not to go out or see people when I can't reliably breathe through my nose, as breathing through your mouth bypasses a lot of the respiratory tract's cilia and mucus cells, so makes you far, far more likely to catch an inhaled virus. But that means cancelling a lot more work than I currently do, which'll be inconvenient to students and leave me short of money, so it's not really that sensible. But I really do not need constant Snot for a couple of months of the year, nor the threat of chronic fatigue due to hyperventilation or resurgent Epstein-Barr. There must be something I can do to improve my immune system, surely?