The holistic philosophy is that if you keep your system strong it will be able to resist whatever is going around. That's why I feed my family a daily cup of chicken soup from Oct - March. (Note add 6 cloves of garlic to chicken soup to enhance immune system.)
Here are some herbs that are known to enhance your immune system.
American Ginseng- supports adrenal glands. Also aids in digestion.
Amla- may prevent and treat connective tissue damage associated with rheumatoid arthritis and lupus
Astragalus - strengthens lungs and good for people with asthma
Cordyceps- helps regulate bloods pressure and strengthens heart
Eleuthero - increases energy, endurance and stamina, improves alertness and cognitive function