Mutating rapidly...that's what a virus does. Cold, flu, whatever, it's constantly changing how it presents to an immune system. That's why there's no general vaccine (use once, always immune) for many viral diseases. The flu vaccine's contents, for instance, are based on an educated guess about what strains will be common during a particular flu season (some years they guess better than others), and don't help much with the following year's flu (a new batch based on a new guess is needed).

Something like the coronavirus will die out when enough humans have had it and recovered, and when it mutates into something that isn't as virulent. The Chinese have data suggesting (though it's early days yet) that such mutation is happening. In the meantime, we need to take reasonable precautions to avoid dumping a huge spike of cases into a US health care system that isn't built to deal with that. The Chinese really do have a better public health system than we do.

And if some guy does get up, cough, and claim to have been in China, there most likely will be a police as well as a health response. That kind of thing isn't a joke under present circumstances.