This study investigates whether the Middle East Respiratory Syndrome (MERS) outbreak in Korea affected online and offline retail sales and determines the presence of a substitution or delay effect between the two. We analyze the monthly retail sales of electronic goods, semi-luxury goods, and groceries using an autoregressive integrated moving average (ARIMA) model with intervention. The findings are as follows. First, offline sales of electronic goods declined by 7.9%, while online sales increased by 7.03%, indicating that these markets can act as substitutes. Second, the offline sales of semi-luxury goods decreased for two months, while online sales remained the same, indicating that there can be a delay effect in the offline market. Finally, despite the slight increase in online sales and the moderate decrease in offline sales, the MERS outbreak did not have a statistically significant effect on grocery sales. Our research findings imply that stakeholders such as the government and retail provided useful information on how to deal with the unexpected outbreak.