Since the SARS-CoV-2 strain of coronavirus that causes COVID-19 first began to circulate at the end of 2019, there has been much debate as to how its symptoms and efficacy at spreading compared to previous outbreaks such as flu, the common cold, and historic coronaviruses such as SARS and MERS. For a short while many were falsely claiming that the death rate and symptoms likened the COVID-19 disease to flu, a dangerous assumption that led to misplaced complacency and was not supported by the mounting evidence for SARS-CoV-2's incredible ability to spread. It's since been more accurately compared to the historic coronaviruses, SARS and MERS.
SARS (severe acute respiratory syndrome) first emerged in the early 2000s. Similarly to COVID-19, it's a coronavirus of zoonotic origin meaning it spread from animals. It's believed to have passed from horseshoe bats to civets before being transmitted by humans. We now know, after close inspection of the novel SARS-CoV-2 pathogen, that it shares many physical properties with SARS, hence its name. Being members of the coronavirus family, both pathogens possess spike proteins, which is how the invasive coronavirus cells are able to attack human cells. SARS-CoV-2 is also thought to have come from bats, with one of the most highly suspected intermediaries currently being pangolins, the most trafficked animal in the world.
Despite the two pathogens' similarities, they are not behaving in the same way as we compare the current outbreak of COVID-19 to that of the SARS outbreak in 2002-2003. A recent and very detailed Twitter thread from Peter Kolchinksy, virologist and author of The Great American Drug Deal, gives an in-depth account as to why the SARS-CoV-2 pathogen is a "nastier" virus than that which caused SARS. The answers reveals why adhering to physical (social) distancing guidelines is so essential in halting the spread of this deadly pathogen.
For clarification, in the tweets, Kolchinsky refers to the original SARS coronavirus as SARS-1 and the COVID-19 coronavirus as SARS-2.
For those skim reading, this is the crux of the differences in the two pathogens: how they manifest and spread, and why the SARS-1 virus was eventually halted.
He also offered up some reassurances from what we've found out about the new coronavirus so far.
The silent incubation period of COVID-19 means that healthy people can be strolling about getting on with their daily lives having absolutely no idea that they are spreading the pathogen as they go. While you might only suffer mild symptoms, or experience no symptoms at all, the further you travel, the more people you speak to, and inanimate objects you touch, the higher the chance you have of passing it on to someone who will suffer severe disease.
Perhaps if you have friends who aren't quite grasping the consequences, the above explanation involving doors and doorknobs might help them to understand.