In a paper published in the British-based journal Nature, the researchers said they had identified a tiny piece of a viral enzyme that does the stealth work and may now become a blocking target for drug designers.

They zoomed in on an enzyme called polymerase, which steers the cell's own organs to crank out viral proteins.

To do this, the polymerase has to slice off a genetic tag, called a cap, from the cell's molecules.

The cap – whose job is to act like an authorisation key, to start the protein-making machinery in motion – is then added to the polymerase.

The cap-stealer is a polymerase sub-unit called PA, according to the study.

"These new insights make PA a promising antiviral target," said Stephen Cusack, head of the European Molecular Biology Laboratory (EMBL) in Grenoble, France, one of several labs who took part in the probe.

"Inhibiting the cleaving of the cap is an efficient way to stop infection, because the virus can no longer multiply. Now we know where to focus drug design efforts."

Annual epidemics of flu result in between three and five million cases of severe illness and between 250,000 and 500,000 deaths every year, according to the World Health Organisation (WHO) website.

These outbreaks are caused by a slight, seasonal drift in the virus' genes.