He said IFITM is a particularly small protein that sits on the surface of vesicles inside cells. Viruses are engulfed by those vesicles and are often destroyed by the cell before an infection can take hold.

Elledge suspects that differences in the amount of IFITM among people might explain differences in their susceptibility to getting sick with the flu. "Natural variation [in this gene] could easily translate into resistance or sensitivity," he said. "We know some people get the flu and are knocked out while others just sniffle a bit."

Drugs aimed at IFITM might fight the flu without an increase in interferon, he said. Interferons are used to combat some viruses such as hepatitis C, but they come with nasty side effects. "You might be able to turn up resistance without making people feel bad," Elledge said. It might even be possible to deliver the protein directly to cells via liposomes. Such a strategy might help in the fight against many infectious diseases, as Elledge showed that IFITM has a similar influence on other viruses, including those responsible for the prevalent mosquito-borne diseases, dengue and West Nile. Likewise, it should be possible to make transgenic animals such as swine or fowl that make more IFITM3 to prevent these animals from becoming reservoirs for flu, where it can recombine with human flu to generate more virulent strains for humans.

The many other proteins they found to be required for the replication of H1N1 could each also yield new ways to fight flu. For its part, IFITM proteins may additionally hold the key to faster production of the weakened virus, as is needed for the making of vaccine.

"If IFITM proteins are also rate-limiting for influenza A virus infection in other organisms such as chickens, whose embryos are employed to passage attenuated viruses for vaccine production, the inhibition of IFITM protein expression could reduce the amount of time it takes to produce vaccine and thereby boost yields," Elledge's team wrote. "This has been a critical issue confounding vaccine production in the current [H1N1] influenza pandemic."