While there has been increasing attention to the role of social media during infectious disease outbreaks, relatively little is known about the underlying mechanisms by which social media use affects risk perception and preventive behaviors during such outbreaks. Using data collected during the 2015 Middle East Respiratory Syndrome coronavirus (MERS-CoV) outbreak in South Korea, this study explores the relationships among social media use, risk perception, and preventive behaviors by examining the mediating role of two self-relevant emotions: fear and anger. The findings demonstrate that social media use is positively related to both of these emotions, which are also positively related to the public's risk perception. The findings also indicate that social media use can significantly increase preventive behaviors via the two self-relevant emotions and the public's risk perception.