A 5-year-old's battle with recurring common colds helped scientists identify why some people are more susceptible to the viral infection than others.

Just weeks after her birth, the young girl started to experience life-threatening colds, the flu, and other respiratory infections. Her doctors suspected she had an immune deficiency, so they went on to do further testing.

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Turns out, she has a rare genetic mutation, according to researchers at the National Institute of Allergy and Infectious Diseases (NIAID). Her case is published in the Journal of Experimental Medicine.

“The human immune response to common cold viruses is poorly understood,” NIAID Director Dr. Anthony S. Fauci said in a statement. “By investigating this unique case, our researchers not only helped this child but also helped answer some important scientific questions about these ubiquitous infections that affect nearly everyone.”

When physicians conducted a genetic analysis, it revealed she had a mutation in the IFIH1 gene that led her body to produce dysfunctional MDA5 proteins, which are vital to the immune system. Normally, MDA5 proteins detect viruses and help fight them off by producing a protective type of protein. However in earlier research, scientists found that mice who didn’t have functional MDA5 proteins were unable to fight against certain viruses.

These findings helped the NIAID researchers conclude that properly functioning MDA5 is necessary to protect against human rhinoviruses (HRVs), the main causes of the common cold.

Read: Why Your Nose Cells Might Hold The Key To A Flu Vaccine, Stopping The Virus Before It Start

In an effort to understand if the IFIH1 mutation affects other people, the researchers looked at more than 60,000 volunteers’ DNA. They found a small number of people have the mutation, but discovered it did not greatly affect them, as most of them lived normal lifespans. The researchers suspect the mutation causes their bodies to compensate in other ways to protect them from life-threatening colds, or that they did get many colds, but didn’t report them. Although the common cold usually isn’t a serious threat, it can require medical attention in people with asthma and other conditions.

“When people have other disease factors, HRV infection can become a tipping point and lead to severe illness, disability or even death,” senior author Dr. Helen Su said. “Now that we better understand the pathway, we can investigate more targeted ways to intervene.”

Every year, colds contribute to more than 18 billion upper respiratory infections around the world, according to the Global Burden of Disease Study. Usually a person recovers from a common cold within a week or 10 days; however, for people who smoke symptoms may last longer. Symptoms vary, but may include runny nose, sore throat, cough, congestion, mild headache, sneezing, and low-grade fever, according to Mayo Clinic.

See also: Could Body Heat Help Cure The Common Cold? Warmer Temps Kill Viral Infections Faster, Study Finds

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