The first step of the viral infection is the union of the spike protein (S) of the virus with (subunit S1) the angiotensin-converting enzyme 2 (ACE2), which contributes to a conformational change in the S2 subunit of the S protein, resulting in the binding of the virus with the transmembrane protein serine protease 2 (TMPRSS2). Here, ACE2 is linked to viral infectious disease.