It is now apparent that SARS-CoV-2 responsible for COVID-19 is a pantropic virus affecting many organ systems and tissues.  Recent research conducted by the National Institute of Dental and Craniofacial Research of the NIH, in collaboration with the University of North Carolina, Chapel Hill demonstrated that the virus infects cells lining the mouth.  Accordingly saliva is a reliable medium to diagnose COVID-19.  Virus in saliva is also derived from nasal drainage and the bronchial and lung expectorate. 

Scientists recovered virus from the salivary glands of deceased patients and confirmed that cells were actively making new copies of virus in material isolated from a living patient.  The saliva of both asymptomatic and symptomatic patients is infectious.

*Huang, N et al SARS-CoV-2 infection of the oral cavity and saliva.  major medicine March 25, 2021 DOI:10.1038/s41591-021-01296-8