The immune system is also bolstered by zinc. If you get low in zinc, the white cells are less responsive. Typically vegetarians have more trouble with this mineral because of the phytates in veggies complex the zinc in them, and prevent absorption. If one needs zinc the best form to use is zinc monomethionine for best absorption. It is called OptiZinc and is not expensive at all.
Males need more zinc than females because it is used in their reproductive tracts.

A good multi-- like Centrum should be enough for most people.
But if you have slow wound healing ...like when bug bites take a long time to heal up etc or cuts on the hands etc, more zinc is a good idea. People who use ACE inhibitors for blood pressure like lisinopril etc, lose zinc from the body.

Don't touch your face when out, pick nose, wipe nose, or rub eyes. Those are the doorways in by viruses. Keep clean tissues with you in public places.

Both WalMart here, and Kroger's provide wipes for the carts by me. And the Library has antiseptic wipes on the counters too.

Vitamin C and EFAs bolster cell walls and help keep viruses out.
Essential fatty acid deficiency results in tiny cracks in the skin and mucus membranes which allow the environmental things to get in. Chronic lung infections/bronchitis are a sign of low EFA status. Flax oil and fish oil (or the new Krill oil) strengthen tissues to resist infection. I used to get bronchitis alot when I was working, being exposed to sick people all day long, but when I started taking the oils, all that stopped!