The PCR tests found SARS-CoV-2 RNA in saliva samples and oral swabs of patients (Chen W et al.; To et al., 2020a; To et al., 2020b; Wölfel et al; Zhang T et al), showing that saliva or mucous can probably transmit the novel coronavirus.  Tests using viral cultures showed that some patients shed viable viruses for 8-9 days and stopped, but some were not tested longer than that and perhaps could have shed longer (Arons et al.; Kujawski et al.; Wölfel et al).  When a person speaks or coughs, that spreads droplets of different sizes into the air, that go in the mouth or nose of another person (Anfinrud et al.). The larger droplets (>5μm, microns, micrometers) in diameter, usually float less than two meters in distance, and fall down onto a surface.  When a person inhales them, they can deposit in and infect the mucous membranes in the nose and bronchial tubes, or the cilia hairs can carry them up and out of the respiratory system (Brosseau; Meselson).   

The Chinese Center for Disease Control and Prevention said that respiratory droplets and close contact probably cause most of the transmission (World Health Organization-China).  Some German medical scientists who examined novel coronavirus RNA from their patients also think that it is spread mostly by droplets in the air (Wölfel et al).