There are several species of echinacea; the most common, and most frequently used in medicinal preparations, is Echinacea purpurea, the purple cone flower.  Echinicia purpurea grows sporadically throughout the eastern U.S. It reaches heights of three feet, with a single, stout, bristly stem. The echinacea flower varies in color from light pink to pale purple. It's a popular ornamental in many gardens. 

The history of echinacea's medicinal use begins with the Plains Indians of the American Midwest, who used it for a variety of ailments, including venomous bites and external infections. In 1887, echinacea was introduced into U.S. medical practice, and it grew in popularity. Indeed, in the early part of the 20th century, nearly every medicine cabinet in the country contained echinacea. However, with the discovery of penicillin and other "wonder drugs," the popularity of echinacea eventually waned. 

The plant was rediscovered in the 1930s by Dr. Gerhard Madaus of Germany, who came to America in search of echinacea seeds. He returned to his homeland and developed a juice from the above-ground portion of E. purpurea. This preparation 
became the most studied and frequently prescribed echinacea preparation worldwide. 

The research of Madaus and others revealed that echinacea contains complex sugar molecules--polysaccharides--that stimulate the cells of the immune system, including white blood cells. Echinacea also supports the production of interferon, an important part of the immune response to viral infections that cause colds and flu. 

Echinacea is available in several different preparations. Again, the most popular worldwide is the expressed juice of the flower. It can also be found in capsules and liquid extracts, frequently combined with Echinacea angustifolia, the herb goldenseal, or vitamin C. 

There are no known side-effects to echinacea, although some believe that it may worsen the effects of auto-immune disorders. Studies looking at this issue are few in number and largely inconclusive. However, until sufficient evidence emerges, echinacea supplements should not be used by those suffering from auto-immune disease. 

Finally, echinacea is most effective when taken intensively for short periods of time. However, some have combined echinacea with other immune tonics like astragalus, reishi, and Siberian ginseng for longer-term herbal programs. When using for the long 
term, the echinacea doses are generally reduced. 
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Brown, Donald J., Herbal Prescriptions for Better Health, Prima Publishing 1996, Rocklin, CA 
Foster, Steven, Echinacea: Nature's Immune Enhancer, Healing Arts Press 1991, Rochester, VT