Other source: It is most likely* caused by a novel coronavirus that either mutated from one of the hundreds of viruses that constantly circulate among humans and return in mutated form once people develop immunity to them or else crossed to humans from another species, both perfectly natural developments that have been going on for millenia and which the human immune system is capable of dealing with.
* I say "most likely" as the existence of a causal relationship between the virus and the disease said to be caused by it has yet to be proven scientifically.