Herbs that support a person’s immune system in the fight against microbes include the following: American ginseng, andrographis, Asian ginseng, astragalus, coriolus, eleuthero, ligustrum, maitake, picrorhiza, reishi, schisandra, and shiitake.

Double-blind trials have shown that common cold symptoms improve^1,2 and recovery is faster³ when andrographis extract containing 48 to 60 mg andrographolides is taken in three or four divided doses daily, beginning as soon as possible after symptoms appear. In addition, preliminary research in Russia suggests andrographis extract may be effective for the treatment of influenza.⁴ This extract was also tested for preventing colds in a double-blind study of teenagers.⁵ After three months, the group taking 5 mg of andrographolides twice daily had only half the number of colds experienced by the placebo group.

Other preliminary⁶ and double-blind^7,8 research has shown similar benefits for treating the common cold from a combination of andrographis extract and an eleuthero extract containing 2.0 to 2.4 mg per day eleutherosides.

Nutrients useful for maintaining healthy immune function are also applicable for preventing infections. Vitamin A plays an important role in immune system function and helps mucous membranes, including those in the lungs, resist invasion by microorganisms.⁹ However, most research shows that while vitamin A supplementation helps people prevent or treat infections in developing countries where deficiencies are common,¹⁰ little to no positive effect, and even slight adverse effects, have resulted from giving vitamin A supplements to people in countries where most people consume adequate amounts of vitamin A.^{11,12,13,14,15,16,17} Moreover, vitamin A supplementation during infections appears beneficial only in certain diseases. An analysis of trials revealed that vitamin A reduces mortality from measles and diarrhea, but not from pneumonia, in children living in developing countries.¹⁸ A double-blind trial for vitamin A supplementation in Tanzanian children with pneumonia confirmed its lack of effectiveness for this condition.¹⁹ In general, parents in the developed world should not give vitamin A supplements to children unless there is a reason to believe vitamin A deficiency is likely, such as the presence of a condition causing malabsorption (e.g., celiac disease). However, the American Academy of Pediatrics recommends that all children with measles should be given high-dose vitamin A for several days.

Vitamin C has antiviral activity, and may help prevent viral infections²⁰ or, in the case of the common cold, reduce the severity and duration of an infection.²¹ Most studies on the common cold used 1 to 4 grams of vitamin C per day.