Their finding could revolutionise the way that scientists understand the virus' genetic structure.

Previously, it has been thought that there are 10 protein molecules produced by the virus which are significant in its activity.

However, scientists from the National Institute of Allergy and Infectious Diseases believe they have found another one.

The new protein was "hidden" in amongst chemicals produced by the virus which are disregarded as "junk" by other experts.

Toxic to cells

However, it is toxic to human cells - particularly human immune cells, which have the job of fighting the virus intruder.

Their suspicion is that the state of the protein could dictate how virulent a particular strain of flu virus is.

Some forms of the virus cause only mild flu - while at the other extreme, others, such as the Spanish flu of 1918, can produce deadly pandemics.

"We believe this is a groundbreaking finding, although we're not yet sure how deep the ground is," said lead researcher Dr Jonathan Yewdell.

"We weren't looking for new proteins at all. We assumed the 10 known influenza proteins were all there were."

Bad copies

When a virus enters a cell, it has the ability to make lots of copies of itself - or replicate, before bursting out to invade many other cells.

The discovery was made when the team were examining "junk peptides" - smaller molecules made by the virus during this replication process.

In the past, these were considered to be simply "manufacturing mistakes" - produced by random errors in the genetic process, and having no role whatsoever.

Dr Yewdell and his team were simply looking to see if the body's immune system had learned to attack any of these small molecules.

In mice, the immune system did attack one of these "junk peptides".

If this were simply a random error like the others, there would be few copies of it, and to check, the scientists used a technique which made it glow green within infected cells.