In early 2020, a new health threat arose with symptoms and mode of transmission comparable to severe acute respiratory syndrome (SARS). Both SARS and the new disease, COVID-19, are caused by coronaviruses and share similar characteristics, especially their severity and high mortality rates and an apparently greater likelihood of transmission. Seasonal influenza is caused by numerous and changing groups of viruses and, for most people, is much less serious. The purpose of this article is to help clinicians compare seasonal influenza, SARS, and COVID-19. It discusses what constitutes best evidence in a time of pandemic, describing the hierarchy of evidence. It is intended to assist dental professionals in providing accurate, current information in answer to patients' questions regarding testing and treatment.