Coronaviruses are a large family of viruses that can cause illness in animals or humans. In humans there are several known coronaviruses that cause respiratory infections. These coronaviruses range from the common cold to more severe diseases such as severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and COVID-19. Like SARS CoV that was first identified in 2002, and MERS which was identified in 2012, Covid-19 is a type of coronavirus that mutated to be able to be able to be transmitted from animals to humans. This important shift makes it a novel, or new, type of Coronavirus that humans hadn't been infected with and therefore had no immunity to. THAT, and its high level of transmissability, made it so easy to spread from person to person, since our immune systems didn't recognize it once Covid-19 was first identified in December 2019. The fact that this Coronavirus resulted in more severe illness was what made the pandemic so devastating.
Think back to 2009 and the H1N1 pandemic. This was also a novel virus. It was also easily spread, although not to the level that Covid-19 is. BUT H1N1 was nowhere close to having the mortality rate that Covid-19 has, which has been the other biggest factor in the Covid-19 pandemic.
Hope that helps! I'm in public health and lead our Covid-19 vaccination initiative so am pretty into the why and how of the pandemic as well as the pros and cons of getting vaccinated
