Coronavirus is just a type of virus so called because the small elevations on it look like crowns (corona means crown). Some of these coronaviruses cause the common cold, but others cause things like MERS. That one type of coronavirus causes minor infections like the cold doesn’t automatically mean that all coronaviruses cause only minor infections. For example, on your post under pox viruses both cowpox (minor) and smallpox (major) are both listed as being caused by the same type of virus, but which sub classification each is in is very important.