Many of those who died from swine flu last year were infected by a mutant version of the virus, researchers have revealed.
The findings at Imperial College, London, UK, helps explain why swine flu proved fatal for some healthy people – but most patients suffered only mild symptoms.
Scientists warned their findings highlight the risk of swine flu continuing to evolve into deadly strains.
The D222G mutation of the H1N1 virus was responsible for about seven per cent of deaths but was found in fewer than two per cent of cases overall.
The new findings suggest the mutation allowed the virus to infect a much wider range of cells in the airway and lungs.
These include ciliated cells, which play a key role in clearing mucus.
The Imperial College researchers worked with the University of Marburg, Germany, on the project.
Researcher Professor Ten Feizi said: "If the mutant virus were to acquire the ability to spread more widely, the consequences could be very serious.
"The study goes to show how important it is that the WHO Global Influenza Surveillance Network continues to monitor closely the emergence of new variants of the flu virus. Even though the 2009 pandemic was relatively mild, it’s vital that we handle outbreaks cautiously and stay vigilant.
"The virus is constantly evolving, and it’s possible that a new form as dangerous as the 1918 pandemic could emerge."
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