Dr Ruslan Medzhitov of the Yale University School of Medicine and his team had identified that anti-inflammatory compounds are produced in response to stress caused by influenza infection.

These are critical to control inflammation, but they cause a profound suppression of the systemic antibacterial immune response, said the authors.

Previous studies have focused on local effects of influenza infection. This is the first to examine the systemic response and susceptibility to secondary infection, the authors claimed.

Dr Medzhitov's team infected mice with influenza virus followed by Listeria monocytogenes bacteria. Samples of the spleen and liver were taken four days after this to detect the presence of viral and bacterial infection.

The researchers' data suggest that lung damage, caused by infection with influenza virus, induces the production of glucocorticoids. These hormones suppress production of the essential cytokine response to bacterial infection.

However, mice unable to produce glucocorticoids, while better able to resist secondary infection, suffered from a lethal inflammatory response.

‘We also found that the induction of glucocorticoids is critical for survival of co-infection,' said the authors.