"All infants are exposed to this and it is potentially preventable," said Dr. Tina Hartert, director of the center for Asthma Research at Vanderbilt University, whose study appears in the American Journal of Respiratory and Critical Care Medicine.

She said it has been known for some time that infants in the Northern Hemisphere born in the fall are at higher risk of developing asthma, but the study is the first to tie this trend to peak viral activity in the winter months.

Hartert and colleagues studied the medical records of 95,000 infants and their mothers in the state of Tennessee.

They found that all babies in the study were at increased risk if they had bronchiolitis, a lung infection usually caused by respiratory syncytial virus or RSV. But autumn babies were at the highest risk.

"What we were able to show was the timing of birth and the risk of developing asthma moves in time almost to the day with the peak of these viral infections each winter," she said.

While genetic risk factors predispose a child to develop asthma, Hartert thinks environmental exposure such as winter viral infection, and particularly RSV infection, may activate those genes.

Nearly every child is infected with RSV early in life, with infections occurring most often between the ages of 3 and 6 months. The virus usually clears up without serious complications.

Hartert said the task now is to prove that preventing such infections could keep infants from developing asthma. "That is where we are now. We need to prove that preventing this infection prevents this lifelong chronic disease," she said.

The easiest way to do that would be a vaccine, but so far, none exists. Vaccine makers GenVec Inc, AstraZeneca's MedImmune unit and others are working on RSV vaccines.