Coronaviruses are very common and symptoms are usually mild because most of them cause upper respiratory infections, meaning they don’t affect the lungs. They get dangerous when a variant comes along like SARS, MERS and this one that can infect the lungs like pneumonia can. My wife has scarred lungs and reduced lung capacity from her bout with pneumonia years ago.
This version is so new I don’t think we know yet how contagious it is from human to human or how lethal it is. My impression is the sooner you get treated, the better your odds of keeping it from your lungs. The progression looks like this: