Winter's coming, and with it yet another nasty cold and flu season. But the germs that seem to increase exponentially with crisper weather and shorter days don't necessarily have to pack a wallop if you effectively arm your immune system with herbs.

A wide range of botanicals, from the Chinese herb astragalus (Astragalus membranaceus) to usnea (Usnea barbata) — a lichen also known as "old man's beard" — has been found to boost immunity against viruses and bacterial infections. But boosting immunity is perhaps too general a term to apply to a system so complex. According to Lise Alschuler, N.D., medical director of Seattle's Bastyr University, "Instead of throwing herbs at someone to increase his or her immunity, the best thing to do is a whole-body analysis to determine what that body needs." Factors that can compromise the immune system include stress, improper diet and a lack of sleep.

Several herbs boast properties that can increase a generally healthy person's resistance against viruses and bacterial infections. Alschuler's top-three favorites for fighting colds and flu are echinacea (E. angustifolia, E. purpurea), osha root (Ligusticum porteri) and black elder flower (Sambucus nigra).

Echinacea, an increasingly popular perennial indigenous to the United States, improves the vitality of the immune system. For centuries, Native Americans have used the herb for snake bites, cancers and infections. Today, many people find that if they take echinacea immediately at the onset of cold or flu symptoms, the severity and duration of the illness is lessened. Most herbalists recommend not ingesting echinacea continuously over a long period of time; rather, alternate it with other system-enhancing herbs, and give your body a rest.

Another herb Alschuler recommends for its immune-boosting powers is astragalus, which can be ingested in dried-powder, capsule form. Or, astragalus bark can be included when preparing soups and teas — especially during the cold winter months.

Other immune-boosting botanicals easily incorporated into diets include garlic (Allium sativum), onion (Allium cepa), gingerroot (Zingiber officinale) and shiitake mushrooms (Lentinula edodes). Garlic, onion and gingerroot — which have been dubbed the "trinity roots" by ayurvedic healing master Yogi Bhajan — are synergistic, so including all three in a soup works well. In large, medicinal quantities (up to 20 capsules daily), garlic can help kick an infection in its early stages. Add shiitake and the lesser-known maitake mushrooms (Grifola frondase) to winter stir-fries, salads and soups for added immune-system stimulation.

Herbal teas are well known for their ability to comfort and soothe; depending on the herbs used, they can also be used to boost immunity and fight colds and flu. Try a cup of tea made with a teaspoon of a dried blend of astragalus, echinacea, licorice (Glycyrrhiza glabra) and ligustrum (Ligustrum lucidum).

If, despite your precautions, you find yourself with a bout of flu this winter, Alschuler recommends an herbal tea bath that stimulates the immune system and kills bacteria: Place two heaping tablespoons of a blend of yarrow flowers (Achillea millefolium), echinacea, elder flower and lemon balm (Melissa officinalis) in 10 ounces of hot water. Steep 20 to 30 minutes, and add to a hot bath. This will help the body create a sweat and rid itself of toxins. Subsequently, sip a tea of elder, echinacea, usnea, astragalus and garlic to speed your body's healing and build your immunity.

Cathy Laws is a freelance writer and editor based in Salisbury, Md.