According to China’s National Health Commission Minister Ma Xiaowei: a person with the Wuhan coronavirus could be infectious to others before they know they have the virus…
At a Sunday press conference, Ma said someone with the virus could show no symptoms they have the coronavirus anytime between a day and 14 days, CBS News previously reported. During that time, an asymptomatic person is also infectious and could spread the coronavirus to others, Ma said, which makes it difficult to stop its spread. [Source]
That statement is in line with this initial study documented in The Lancet. That same study, was also the first confirmation of person to person transmission – and I highly recommend that you read it.
While a few of the medical terms might be foreign to you – you really should take this opportunity to educate yourself. The Chinese doctors that wrote the study did a fantastic job laying out all the symptoms, and contact tracing.
UPDATE (30 Jan): There has been an asymptomatic case in Japan and Canada already — following the initial confirmed asymptomatic case in China. The only question is: are they shedding virus while asymptomatic.