The standard test for COVID-19 diagnosis is real-time reversetranscription-polymerase-chain-reaction (RRT-PCR). However, RRT-PCR is not readily available and demonstrates moderate to high rates of false-negative results.
Despite imaging not being a tool for COVID-19 diagnosis, there has been an increased number of chest
computed tomography (CT) scans done worldwide to assess the severity and extension of lower respiratory tract involvement. Plain chest radiography is often inaccurate in initial stages of the infection, while high-resolution CT is extremely sensitive for diagnosing ground-glass opacities (GGO).
There is no consensus on the use of chest x-ray or CT of suspected COVID-19 pneumonia, and the choice of imaging modality will depend mainly on the physicians’ judgment. The following classification of COVID stages according to the interval between the onset of symptoms and the CT scan: early phase, 0 to 5 days; intermediate phase, 6 to 11 days; and late phase, 12 to 17 days.
In the early phase, the disease is mainly characterized by single or few rounded low-density GGO with a
subpleural location. However, it is worth emphasizing that 11-15% (sporadically up to
approximately 50%) of patients may have normal CT scans 0-2 days after onset of flu-like
symptoms from COVID-19. 
Phylogenetic analysis reveals that SARS-CoV-2 is closely related to SARS-CoV in the Sarbecovirus subfamily (genus Betacoronavirus). While an intermediate host has yet to be determined, it shares strong genetic similarity (>95%) to known bat coronaviruses from China, suggesting a likely bat origin. Relatively similar coronaviruses have been found in pangolins, but in these the receptor-binding domain (RBD) of the spike (S) glycoprotein is more like that of SARS-2-CoV-2 than that of known bat viruses. 
- Coronavirus disease 2019 (COVID-19) pneumonia presentations in chest computed tomography: a pictorial review
Hochhegger B Mandelli N Stüker G Meirelles G Zanon M Mohammed T Verma N Sverzellati N Marchiori E; Current Problems in Diagnostic Radiology
- COVID-19 pandemic—a focused review for clinicians, Cevik M Bamford C Ho A, Clinical Microbiology and Infection, Publisher: Elsevier B.V. 2020 vol: 26 (7) pp: 842-847