Catching a cold usually isn't a big deal, but one 5-year-old girl's her recurring colds end in hospitalization. The child, whose case was described Monday in the Journal of Experimental Medicine, has a rare mutation that stops parts of her immune system from functioning properly. By studying her case, scientists have a better idea of how our body fights off the cold and related viruses.

Why it matters: According to Su, the average adult gets 3-4 colds a year, and they usually recover with no problems. But sometimes they don't, and it is unclear why. "That's why it's really important to study cases like these. It's hard to tell when some viruses are dangerous and when some won't be."

What they found: The mutation is in a gene that codes for a protein called MDA5, which is a part of a body's first line of defense against intruders. It's particularly good at identifying viruses similar to the common cold as well viruses in the same family as MERS and SARS. Although MDA5 has been studied extensively in mice and in the lab, not many humans with MDA5 mutations have been identified, says study author Helen Su, an immunologist at the National Institute of Allergy and Infectious Diseases. She and her team were able to directly link the protein to infections with the cold virus.

Go deeper: The child described in the case study isn't the only person with a MDA5 mutation. Studies have identified individuals with less-than-perfect MDA5 genes, and many of them are healthy. In fact, some mutations of MDA5 may even decrease the likelihood of type 1 diabetes.