Six reasons they’re so harmful, and one reason they’re less infectious than other diseases. By Dan Werb Dr. Werb is an epidemiologist. March 20, 2020, 3:00 p.m. ET Credit In the past two decades, coronaviruses have produced three deadly global epidemics: SARS, MERS and now Covid-19. If you’re wondering what it is about this family of viruses that makes it produce such deadly pathogens — and what about them might hinder their spread — here are a few things you should know. 1.