For many, the novel coronavirus that started in China’s Hubei province and is now sweeping around the world came as a surprise. For experts on epidemics and pandemics, it did not: 2019-nCoV is ‘Disease X,’ a worrisome virus of unknown origin with no known treatment that they knew would one day emerge. Epidemiologists, infectious disease clinicians, pandemic preparedness advocates and others have spent years anticipating an outbreak of an entirely new infectious disease, and they have insights and answers for us in this rapidly evolving public health emergency.