Although cathelicidin has yet to be studied directly on influenza, recent research has shown that it attacks a variety of fungi, viruses, and bacteria - including the bug that causes tuberculosis, researchers reported last March in Science.

There's also indirect evidence of the sun's role in seasonal flu, Cannell and his colleagues argue.

In the late 1970s, Soviet scientists inoculated nearly 835 young men in St. Petersburg with a weakened form of the influenza virus during different seasons of the year.

The men, they discovered, were eight times more likely to develop fever and other signs of flu infection in winter than in summer. Repeating the experiment in another city yielded similar results.

And what of the sun-drenched tropics?

Cannell and his colleagues point to a scattering of studies showing vitamin D deficiencies even in Hong Kong and other equatorial spots.

Another important clue: A 2003 review of tropical influenza infections found the virus strikes mainly during the rainy season, presumably a time of reduced sun exposure.

Maybe yes, maybe no

Some researchers find the Vitamin D theory tantalizing. "It's a wonderful story, and I think there's considerable substance behind it," says immunologist Dr. Michael A. Zasloff, a professor at Georgetown University.

Others, however, wonder whether this is just another vitamin fad - just as vitamin C was once promoted by double Nobel laureate Linus Pauling as a cure for the common cold.

"They have manipulated the literature (some of it very bad literature) to prove their points," Dr. James Cherry, a pediatric infectious disease specialist at UCLA's David Geffen School of Medicine, wrote in an e-mail.

However, "The hypothesis should be easy to prove or disprove with a controlled, blinded study," he added.

Vitamin D isn't the only substance being kicked around as a seasonal stimulus candidate.

The CDC's Scott Dowell favors melatonin, a naturally occurring hormone whose production is tied to the daily light-dark cycle. Preliminary research suggests that the hormone might help regulate the body's immune response.

"I don't know where it's going to lead," he concedes.

Cannell, meanwhile, says he takes 5,000 IU of vitamin D during the winter months, more than twice the maximum daily recommended dose.

He says he rarely gets sick anymore.