At this very early point in the emerging 2019-nCoV outbreak, knowledge about the virus is insufficient to predict what shape that outbreak will ultimately take. But knowledge about the virus is accumulating at remarkable speed, and experience with other viruses is helping to shape the response to the newest coronavirus threat. 2019-nCoV, sometimes called Wuhan coronavirus after its source, is the third coronavirus after SARS-CoV and MERS-CoV with the potential to cause serious illness and death that has emerged since the beginning of the 21st century.