Whether cold temperatures have anything to do with catching a cold has long been a question that supposedly separates believers in old wives' tales from the scientifically savvy. But while the cold-cold connection is widely considered a medical myth, a new study finds otherwise.

Even a slight chill increases the speed at which rhinoviruses, which cause the common cold, multiply in lab mice, said the study published on Monday by Yale University scientists. Cold temperatures also trigger immune-system changes that let the viruses replicate virtually unchecked.

Scientists have suspected for more than half a century that rhinoviruses thrive in a slight chill. A 1960 study found that they multiply more quickly at 33 degrees Celsius (91.4 Fahrenheit) than at body temperature (37C, or 98.6F.).

The new study, published in Proceedings of the National Academy of Sciences, confirmed that finding, showing cold viruses replicated more efficiently and produced higher levels of infectious particles at the lower temperature.

But it also extended the 1960 results, pinpointing three biological effects of chilly air that can increase the likelihood of developing a cold.

In cells lining the mice's nasal passages, genes that produce the virus-fighting protein interferon were less active at 33C than at 37C, Yale immunologist Akiko Iwasaki and her colleagues reported.

In addition, molecules that detect viruses inside cells and then order the cell to produce interferon were less sensitive at colder temperatures. That lower sensitivity reduced production not only of interferon but also of proteins that chop up virus genes, block the release of virus and kill virus-infected cells.

Exposure to a rhinovirus is still a prerequisite for catching a cold. But once a few viruses have entered cells of the nasal cavity, Iwasaki said, inhaling cold winter air exposes those cells to the chill "that the virus likes to replicate" and causes the immune system to respond less aggressively.

"Altogether," she added, "these temperature effects can result in an 100-fold difference in the level of cold virus" at 33C compared with 37C after three days - enough to turn an asymptomatic viral population into sneezing, runny-nosed misery. – Reuters