But what is the scientific evidence for alternative cold and flu remedies, such as echinacea, ginseng and zinc? Dr. Ronald B. Turner, a cold-virus expert at the University of Virginia School of Medicine in Charlottesville, says that the evidence is weak. But he doesn't try to discourage people from spending their money on remedies they think make them feel better, as long as there are no harmful side effects.

"One of the things that permits people to believe in unproven therapies is that it really doesn't matter what you take because you're going to get better anyway," he said.

Symptoms of the common cold increase rapidly and disappear just as fast, Turner said, making it difficult, but not impossible for scientists to design studies proving what works.