During the outbreak of 2009-2010 H1N1 or "swine flu", the same virus that caused mild coughing and sneezing in some patients proved deadly for others. It highlighted a medical mystery: why are some people more fit to handle the flu than others? British and American researchers think they have a clue. Reporting on Sunday in the journal Nature, researchers say they've found a gene that influences our ability to fight against flu illness. The gene, called IFITM3, is the "important first line of defense" against the flu, researcher Paul Kellam of Britain's Sanger Institute told Reuters. The IFITM3 protein  prevents flu viruses from reproducing in cells. So, if you have a high amount of IFITM3, you're in luck, since it weakens the spread of the virus; a low amount can lead to more rapid viral reproduction, causing severe flu symptoms, according to Kellam. Scientists first found in gene studies that IFITM3 played a role in resistance to flu and other viruses. In experiments in mice, they showed that when mice were raised to lack the IFITM3 gene and then infected with influenza, the animals developed more severe lung infection from flu, compared with mice that had the gene. To figure out the role of IFTTM3 in humans, researchers then studied the IFITM3 genes of 53 patients with the flu in 2009-2010. They found that these patients were more likely to have a variant of the IFITM3 gene -- which makes people more vulnerable to flu -- compared with the general population. According to evidence from large genetic databases, about 1 in 400 people carries this IFITM3 variant, the BBC reports. "Our research is important for people who have this variant as we predict their immune defenses could be weakened to some virus infections," said Kellam. The Findings also suggest that a drug that functions as IFITM3 could help increase resistance to flu and other viruses, including deadly ones like H5N1 flu. You are less likely to get infected with H1N1 if      _
a variant of EFIM3 is produced the immune system is weakened protein doesn't function in your cells you have a high amounts of IFITM3