Let's first think of a practical example. 

Farmers have been using pesticides for a considerably long time. Over time, insects developed resistance to the poison. This escalated to such high levels that the best pesticide of those days, DDT(Dichloro Diphenyl Trichloro ethane) had to be banned for 40 years since all insects developed resistance against DDT.

Now back to the point, how did Corona Virus become such a strong virus with extremely stable Protein structure ?

Bats have one of the most efficient  immune system against viruses. So any virus infecting bats wont really sustain for a considerably long time in its body. Neither can it do any damage to the Bat nor its immune system. So due to harsh environment conditions to live under, viruses have been updating their protein structures regularly. This counter move by the viruses has made them more stable and highly fatal over time. 



So when a bat bites an animal or a human, the virus in the fangs get into the hosts body and finds the immune system of the animal to be extremely weak compared to the Bat's. In no time they multiply in numbers making it almost impossible for the poorly developed immune system to even fight back. Even if the system tries to fight back huge inflammations are observed in the hosts body. To be practical its like asking a very well warmed up Virat Kohli to face gully bowlers.

Was a Chinese man bitten by a Bat ? No, infact the bat was bitten by the Chinese as a delicacy - Bat Soup. 

The Corona has ended up to be so strong that it can sustain in open environments like door knobs for around 9 days. It can also transfer via a bad breathe.

 So can the masks really be of any help if the virus can sustain for so long on any surface ? That's like running away from a hungry lion on a cycle, but yes that's better than a walk.