The extinct influenza virus that caused the worst flu pandemic in history has been recreated from fragments of avian flu found in wild ducks in a controversial experiment to show how easy it would be for the deadly flu strain to reemerge today.

Scientists said the study involved infecting laboratory ferrets with close copies of the 1918 virus – which was responsible for the Spanish Flu pandemic that killed an estimated 50 million people – to see how easy it can be transmitted in the best animal model of the human disease.

But other researchers have denounced the research as foolhardy and dangerous. Critics said that any benefits of the attempts to recreate 1918-like flu viruses from existing avian flu strains do not justify the catastrophic risks if such a genetically engineered virus were to escape either deliberately or accidentally from the laboratory and cause a deadly influenza pandemic.