Rhinovirus, the most frequent cause of common colds, can prevent the flu virus from infecting airways by jumpstarting the body's antiviral defences, suggest the findings of a recent study. The study was published in the journal The Lancet Microbe. The findings help answer a mystery surrounding the 2009 H1N1 swine flu pandemic. An expected surge in swine flu cases never materialised in Europe during the fall, a period when the common cold becomes widespread. A Yale team led by Dr Ellen Foxman studied three years of clinical data from more than 13,000 patients seen at Yale-New Haven Hospital with symptoms of respiratory infection. The researchers found that even during months when both viruses were active if the common cold virus was present, the flu virus was not. To test how the rhinovirus and the influenza virus interact, Foxman's lab-created human airway tissue from stem cells that give rise to epithelial cells, which line the airways of the lung and are a chief target of respiratory viruses. They found that after the tissue had been exposed to rhinovirus, the influenza virus was unable to infect the tissue.