ACE2 and COVID-19: A plausible biological explanation for the association of GD and PD with COVID-19 risk and severity is the increase in the expression of viral adhesion receptors such as ACE2 on the mucosa of oral cavity, galectin-3 (44–48) and transmembrane protease serin 2 (TMPRSS2), on oral epithelium which is necessary for activation of the SARS-CoV-2 S-protein to bind with host cells and increase its infectivity (49, 50).