Nov. 21, 2008 -- When it comes to babies and asthma, timing is everything.

A new study shows that babies born four months before the peak of winter virus season are more likely to develop childhood asthma than babies born at any other time of year; that's because the timing increases the chance of a viral respiratory infection during infancy, which in turn increases the risk of childhood asthma. The date that winter virus season peaks can vary from year to year.

The study is published in American Journal of Respiratory and Critical Care Medicine.

Asthma is an increasingly important health concern. The prevalence of asthma increased 100% total between 1985 and 2001, according to background intelligence in the study. About 300 million people have asthma. Deaths from asthma are expected to increase 20% during the next decade.

Researchers looked at medical records of 95,310 children born between 1995 and 2000 and followed their health status until 2005. The children were all born in Tennessee and enrolled in the state's Medicaid program, called TennCare.