It looks like the course of disease progression could be unpredictable, with patients who intially seemed to not have too harsh a case of the disease taking sudden turns for the worse after days or even a week of mild symptoms. 10% of patients having abnormal initial symptoms would also make control of the disease a lot harder if 10% of people escape your screens.
Source: https://www.nytimes.com/2020/02/07/health/coronavirus-patien...