The basic way our body responds is that viruses have a specific set of RNA which our body defends against and creates memory cells + antibodies to detect that specific RNA and attack the same virus if it attacks you again. Now, if you haven't noticed - "common cold" might occur again even though you have recovered from it, why? Because each time the virus has different RNA which our body can't detect thus it uses all of our immune response in order to fight the virus, in case of same RNA the virus immediately gets wiped out by our memory cells which is created by our immune system in order to fight that virus that has the same set of RNA.
Issue here is that "corona virus" might be having a different set of RNA each them they infect a victim, in order words our body just can't tell if it's the same foreign organism or not, thus it just does what it needs to do - fire up our immune responses and try to fight the infection. Think of RNA as characteristics of how it defines the virus, I am telling you whenever I see people saying something like "I have got a strong immune system", it kind of makes me laugh.
See Ebola for example, it was a virus which never got any vaccine made. The more stronger your immune system was, the more strong the immune response will be which ultimately lead to death (This is what Ebola made use of) as the body self-destructed itself because it had no other options left. Yes, having a strong immune system is also something that is not reliable. Be it strong or weak, these viruses have a way of confusing our body's flawed immune system.
Corona is just a new name for the same 
