It's not really a 'mystery virus' though some media have been calling it that. It's a previously unknown ("novel") coronavirus. But they have sequenced it and we understand it . . . there's already a diagnostic test available.
It is picking up pace though. We're now up to over 200 cases in China and a handful abroad including in Thailand, Japan, and ROK (South Korea) - probably elsewhere, as the city in which it originated (Wuhan) is a transportation hub.
Three people have now died from the Wuhan flu. It's still too early to get reliable mortality rates but it seems lower than SARS. Coronaviruses, however, are believed to mutate easily so a novel coronavirus with a propensity to be lethal always trouble epidemiologists greatly because transmission and mortality can seem manageable for a while but a mutation to a more dangerous virus is always possible.