Last year, we talked about whether or not Grandma is right when she tells you to bundle up when you go outside, lest you catch a cold. We went over the fact that colds and the flu are caused by viruses, but that there was a little bit of evidence suggesting that being cold can contribute to getting sick and make it easier for a viral infection to occur.

Now we have a little more.

Last week Nature reported on new research from a team at Yale University, who presented their findings at the annual meeting of the American Society for Microbiology. In experiments with both mice and lab-grown human cells, the researchers found that low temperatures weaken immune system defenses against cold-causing rhinoviruses. Beth Mole explains:

Foxman and her colleagues at Yale studied mice susceptible to a mouse-specific rhinovirus. They discovered that at warmer temperatures, animals infected with the rhinovirus produced a burst of antiviral immune signals, which activated natural defenses that fought off the virus. But at cooler temperatures, the mice produced fewer antiviral signals and the infection could persist.

Next, the researchers grew human airway cells in the lab under cold and warm conditions and infected them with rhinovirus. They found the warm infected cells were more likely to undergo programmed cell death — cell suicide brought on by immune responses aimed at limiting the spread of infections—than the cold-infected cells.

Foxman says the data suggest that these temperature-dependent immune reactions help to explain the virus’s success at cooler temperatures, and why winter is cold season. As temperatures drop outside, humans breathe in colder air that chills their upper airways just enough to allow rhinoviruses to flourish, she says.

This adds to a whole pile of ideas scientists have about why cold and flu infections spike in the winter, some tied to temperature, others to human physiology and still others to our behavior (e.g., staying indoors when it's cold out and making viral transmission very easy). Whether these causes work alone, simultaneously but separately, or in combination with each other still needs to be worked out.