RE: teachers and others who work with the public: what ...
Definitely stock up on alcohol-based hand sanitizer (the alcohol-based ones don't risk creating resistant bacteria the way the Triclosan ones do): keep a small one in your pocket or purse, one on your desk. And after you've used the rest room and washed your hands there, go back to your room or whereever and use the sanitizer (I'm amazed at how many college students don't wash their hands after using the 'facilities,' so my washing my hands, then touching the doorknob they just touched, doesn't do the trick).
Drink plently of fluids (which keep the mucus membranes moist so they can do their job better) and use a humidifier to keep the air humidity good, especially at night while you sleep.
I felt a bit 'fluey' several weeks ago, and started taking a morning smoothie with açai and a whole foods vitamin C powder made from dried acerola cherries (vitamin C works best in concert with the synergistic elements it comes with in nature, like bioflavnoids, as those found in real fruits). It nipped that bug in the bud.
Other immune system boosters: garlic and onions, echinacea (shouldn't be taken for long periods), astragalus (has no known adverse effects, so can be taken daily--several immune support herbal formulas contain it. Astragalus is probably the best of the bunch to use continuously.