For some, autumn is a season of vibrant treetops and sweet-smelling pumpkin pie. For others, it's a time of stuffy noses, migraine headaches, and unshakable chest coughs. When the weather changes, it seems that everybody comes down with a cold. No scientist has discovered a cure for the common cold, but many believe that taking by-products of a cone-shaped plant can help make you less susceptible to catching one.

The plant? Echinacea. Resembling a black-eyed Susan, the echinacea plant is a North American perennial indigenous to the Central Plains. Though some physicians still view it as "alternative" medicine, 20 years of clinical research has proven that, on some level, echinacea is effective at increasing the body's resistance to colds and other infections. Many of these studies indicate the herb helps the body do the following:

Last year, a Swedish study proved that nearly half the patients who took echinacea at the first sign of a cold avoided developing a full cold, and that those who took the herb recovered more quickly than those who did not.

With so many potential benefits, you might think it best to pop echinacea every day, like a multivitamin. However, research also indicates that the herb works most effectively when taken sparingly. In 1987, a German study found that a single dose of the herb stimulated the immune system in humans, but that taking it repeatedly over a period of days actually suppressed it.

Much of the research on echinacea is inconclusive. While it appears that taking echinacea helps patients fight colds better, scientists who study the herb are quick to note that its effectiveness requires further evaluation. "There's still a lot to learn," says Mindy Green, director of education for the Herb Research Foundation, in Boulder, Colorado. "We know it works, but answers to questions about how and why are still somewhat unsure."

According to Green, scientists have proven the herb prevents the formation of a hyaluronidase, an enzyme that destroys the natural barrier between healthy tissue and unwanted pathogens. They believe fat-soluble alkylamides and a caffeic acid known as echinacoside also contribute to the herb's effects on immunity.

Though much of the research on echinacea is relatively new, the herb itself dates back to the 1600s. For centuries, American Indians placed it on skin wounds and snakebites. Then, in the 1870s, a German doctor visiting in the United States promoted the herb as a blood purifier and as treatment for headaches, rheumatism, indigestion, and hemorrhoids. Until the 1930s, it was used to reverse the effects of dyptheria, typhoid, and meningitis. Earlier this decade, European physicians promoted the herb in the treatment of urinary tract infections and herpes.

Today, nine species of echinacea grow naturally in American soil. Echinacea purpurea, Echinacea angustifolia, and Echinacea pallida are among the most common, and these plants are usually crushed or ground into juice for easy ingestion and application. Since 1995, echinacea has become one of the most popular herbs in the United States, ranking as the top-selling herb in the health food market. What's more, nearly 100,000 pounds of wild echinacea are harvested and shipped overseas every year, making echinacea one of the most sought-after native herbs in American history.

According to Mark Blumenthal, founder and executive director of the American Botanical Council in Austin, Texas, this is perhaps the herb's most enduring legacy. "We often get caught up with the exoticness of using rain-forest plants from the Amazon, and we have a tendency to overlook the medicinal plants from our own backyard," Blumenthal says. "Echinacea is a good example of a native herbal medicine that works."

No significant side effects have been reported with the use of echinacea, though the herb's toxicity has yet to be carefully examined in long-term clinical trials. German health authorities recommend that no one take it (internally or externally) for more than eight weeks in a row. Blumenthal says you should not take echinacea if you suffer from severe allergies to ragweed and adds that you should always consult your doctor before mixing medicines of any kind.

Echinacea Dos and Don'ts

To achieve echinacea's maximum benefit, here are some tips from Mark Blumenthal, founder and executive director of the American Botanical Council.