Scientists are detecting novel RNA in multiple patients with influenza or pneumonia -like conditions, and are assuming that the detection of RNA (which is believed to be wrapped in proteins to form an RNA virus, as coronaviruses are believed to be) is equivalent to isolation of the virus. It is not, and one of the groups of scientists was honest enough to admit this:
"we did not perform tests for detecting infectious virus in blood" 
But, despite this admission, earlier in the paper they repeatedly referred to the 41 cases (out of 59 similar cases) that tested positive for this RNA as, "41 patients… confirmed to be infected with 2019-nCoV."
Another paper quietly admitted that:
"our study does not fulfill Koch's postulates" 
Koch's postulates, first stated by the great German bacteriologist Robert Koch in the late 1800s, can simply be stated as:
- Purify the pathogen (e.g. virus) from many cases with a particular illness.
- Expose susceptible animals (obviously not humans) to the pathogen.
- Verify that the same illness is produced.
- Some add that you should also re-purify the pathogen, just to be sure that it really is creating the illness.