Scientists at Trinity College Dublin have discovered that a biological molecule is also critical in protecting against infection.

They say STAT3 is vital, as people would be unable to fight the common flu virus without it.

Their discovery could pave the way for new methods to restore immunity against a whole range of viruses.

The team behind the work was led by assistant immunology professor Dr Nigel Stevenson.

The findings have been published in the journal Cellular and Molecular Life Sciences.

Researchers say with newly emerging viral infections - such as SARS, ZIKA and Ebola - the importance of understanding how viruses target our immune system, and the need to develop new protections, has never been greater.

During any viral infection our cells produce an immune molecule called Interferon, which essentially 'interferes' with the attacking viruses - preventing them from replicating in our bodies.

When our cells are stimulated by Interferon a cascade of molecules within our cells is activated, like a series of dominos.

When the final one falls, the cell should be able to clear the viral infection. This cascade is known as a signalling pathway as it passes the 'danger signal' of viral infection through the cell.

The Interferon signalling pathway produces several hundred immune molecules that act to destroy viruses and increase our immune response.

Immunologists from Trinity, wondering how viruses suppress the immune response, have discovered that they have evolved numerous methods to stop these signalling pathways, thereby blocking responses to Interferon.

Dr Stevenson said: "We thought that since the Interferon signalling pathway enhances the immune response against viruses so effectively, viruses might have evolved means to block it - such a reality would explain why several viruses are so troublesome to defeat.

"For decades we have known that STAT3 is essential for healthy cell growth, but our new revelation identifies it for the first time as an essential anti-viral component in the Interferon signalling pathway. In fact, we found that without STAT3, cells cannot fight the common Flu virus or the Pox vaccinia virus.

"Of course a major goal of our ongoing work is to find solutions to the real-world problems faced by the thousands of people who cannot clear certain viruses after they have been infected.

"This discovery opens the door to new therapeutic options, which, we hope, will be able to help people restore their natural immunity against a host of problematic viruses."

Scientists had, until now, believed they fully understood how the Interferon signalling pathway worked.

But by using a series of viral infections and molecular techniques, Dr Stevenson's team discovered a new anti-viral role for STAT3.