European scientists on Wednesday said they had uncovered how the influenza virus is able to take over human cells and use them as machinery to replicate itself.
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.
A more alarming change, though, is when the virus picks up novel genes, becoming a pathogen to which no one has immunity.
This happened in the 1918-9 pandemic of so-called Spanish flu, in which tens of millions of people were killed.
It also drives the anxiety for H5N1 avian influenza, or "bird flu," a new strain that can be transmitted to humans from poultry but, at present, is hard to be handed on from human to human.
Health watchdogs have called for new drugs to combat the threat, as there are only four frontline treatments in the antiviral arsenal.
Last year, the same European team identified another key part of polymerase, a subunit called PB2 that recognises the "cap" and then binds to it. The cap is then chopped off.
"Taken together, the two findings provide a close-to-complete picture of the cap-snatching mechanism that allows the influenza virus to take control over human cells," EMBL said in a press release.