ACE2 and infection: Moreover, SARS-CoV-2 is transmitted and pathogenic through the oral mucosa, as the main host cell receptor for COVID-19, angiotensin-converting enzyme II (ACE2) plays a crucial role in the final infection by virus entering cells [16, 17], and there is also a high expression of ACE2 in the oral mucosa (primarily in tongue epithelial cells) [18].