Why the flu makes some people sicker than others

The flu can confine even those with the strongest immune systems to bed. And anywhere from 3,000 to 49,000 people die from the virus every year. Why does the flu prove deadly for some people, and not others? That's the question an international team of researchers answers in a new report in the journal Nature. Lead researcher Aaron Everitt says his team discovered that the absence of one gene is what could cause the flu to be fatal. Here, a guide to a genetic flaw that causes influenza to hit some harder than others:

What is the gene?

It's called IFITM3, and it's the first line of defense against the flu because it emits a protein that limits "the spread of viruses in cells," particularly in the lungs, says Everitt. Although most people have an abundance of IFITM3, some have dangerously low levels.

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How did researchers identify it?

Researchers suspected that IFITM3 was crucial to resistance against the flu and other viruses like dengue fever and West Nile. To test the hypothesis, they removed the gene from mice, and then infected them with the flu. The gene-deficient animals "developed more severe respiratory and lung function from the flu, including pneumonia, compared to mice that had the gene," says Alexandra Sifferlin at TIME.

Did they test human genes?

Yes. For the next step, researchers sequenced the IFITM3 gene of 53 patients who were hospitalized with the flu in 2009 and 2010. One in 18 patients were found to have a mutation of the gene, which, although "rare for normal people," might make them potentially more vulnerable to influenza's onset of symptoms, says Red Orbit.

So how can we use this information?

In the future, identifying the gene could help doctors screen for patients more likely to be "brought down by flu, allowing them to be selected for priority vaccination or preventative treatment during outbreaks." In 2009 and 2010, for example, during the H1N1 pandemic, most patients experienced mild symptoms, while some otherwise healthy young adults became ill and died, says Kate Kelland at Reuters. Knowing who's more vulnerable during a flu outbreak would make saving lives a lot easier.

Sources: BBC, Red Orbit, Reuters, TIME