Coronaviruses are a group of viruses that have been known about for a long time. It’s the same family that caused the SARS outbreak in 2002 and it’s official name now is SARS CoV-2.
This is the most recent mutation of a Coronavirus. The change is of the virus rather than our immune systems. It’s more dangerous than SARS was as although it seems less deadly its symptoms are milder meaning that it will spread more easily and infect more people. A lower death rate but with more people infected it has more chances to kill.