A new study published in PLoS Pathogens found “that a gene mutation could explain why some people are more susceptible to developing” cold sores than others. While 90 percent of people are infected with the herpes simplex virus type 1, “only about one in five people actually develop sores.” University of Edinburgh researchers “found that a mutation on the IL28b gene makes it so the body’s immune system is not able to prevent the...virus...from becoming active, thereby causing the cold sore to develop.”