In line with the periodontitis–nosocomial pneumonia association discussed above, the aspiration of periodontopathic bacteria induces the expression of angiotensin-converting enzyme 2 (ACE2), a receptor for SARS-CoV-2, and after cleaving the S protein (essential for the development of SARS-CoV-2) by producing proteases [50], may link periodontitis with COVID-19 (Figure 1). Here, PROS1 is linked to periodontitis.