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The return of cooler weather usually marks the beginning of another flu season, which runs from November to March. But medical experts have never been certain why influenza always strikes at the same time of year and disappears with equal predictability.

Some have suggested the flu easily spreads when we all crowd indoors. Others have speculated that we get rundown during the punishing winter months, leaving us more vulnerable to infections.

Now, however, a team of researchers claims to have finally solved the mystery - and it has little to do with human activities. Instead, the virus just seems to thrive when it's cold and dry, says lead researcher Peter Palese at Mount Sinai School of Medicine in New York.

For their study, the researchers put infected and uninfected guinea pigs in cages and then altered the temperature and humidity levels.

According to the results published in the journal PLoS Pathogens, the virus spreads more easily at 5 C than at 20 C, with no transmission at 30 C. A low relative humidity of 20 to 30 per cent also provides ideal conditions for the virus to flourish.

Influenza is normally spread through the air when an infected person coughs or sneezes. But why would a cooler and drier environment assist in the transmission of the bug?

Dr. Palese said the virus appears to be more stable at lower temperatures. "It is like refrigeration. It lasts longer," he said. Cool air also tends to increase the production of nasal mucus, which can act as a secure foundation for the virus to take hold and spread in the respiratory tract.

And dry conditions help the virus remain airborne longer, increasing the chances it will be breathed in by some unsuspecting victim. In the journal, the researchers conclude: "Influenza virus transmission indoors could potentially be curtailed by simply maintaining room air at warm temperatures (more than 20 C) and either intermediate (50 per cent) or high (80 per cent) relative humidity."

But Dr. Palese emphasized in an interview that the annual flu shot is still the best way to fend off infection.

MS study in the works

Canadian researchers are embarking on a major study to find out whether a common antibiotic can slow down the advance of multiple sclerosis.

Previous research already suggests that minocycline could be a promising option for people in the early stages of the disease.

MS is a disorder in which the patient's own immune system attacks and inflames myelin, the protective covering on nerves. As the disease progresses, it can affect vision, hearing, memory and movement.

Minocycline, which is often used to treat acne, has unique properties that make it well suited to the fight against MS. In particular, it inhibits the activities of an enzyme and immune cells that are key to initiating MS attacks.

"It has many biologic actions," said lead researcher Luanne Metz of the University of Calgary.

Just as important, it's relatively cheap. Because it is available in a generic pill, an annual supply can be purchased for as little as $800. Current MS therapies cost substantially more - between $18,000 and $40,000 a year.

The researchers hope the use of minocycline might delay the rate of disease progression, so patients wouldn't need the costly drugs until a long time after their initial diagnosis.

The $4-million study, financed by the Multiple Sclerosis Society of Canada, will involve 200 patients at 14 medical centres across Canada.

Hold that stretch

Do you stretch before or after a physical workout, hoping to reduce post-exercise muscle soreness? A new study suggests you are wasting your time.

Australian researchers reviewed 10 separate trials and found little evidence that stretching can prevent muscle soreness in young healthy adults. In fact, the various trials indicate that stretching reduced soreness by less than one point on a 100-point scale.

"The practice of stretching before or after exercise may have arisen because of the mistaken belief, prevalent in the 1960s and 1970s, that muscle soreness was due to muscle spasm," the researchers write in the Cochrane Library, a publication of an international organization that evaluates medical studies.

"The muscle spasm theory of muscle soreness has since been discredited, but the practice of stretching persists."

But even with this latest study, exercisers are not expected to change any time soon. "I personally believe it will take a long time for people to actually believe that stretching doesn't help to prevent muscle soreness after exercise," Marcos de Noronha, a PhD student and one of the researchers at the University of Sydney, said in an e-mail interview.

Does this mean that regular stretching has no benefit for anyone? Not necessarily. The researchers say that further trials are needed to assess "long-term stretching programs in community-based populations and for people with reduced flexibility."

ptaylor@globeandmail.com