The S protein consists of two distinct subunits: the S1 subunit, which binds to the known host receptor, angiotensin-converting enzyme 2 (ACE2) (2, 3, 4, 5, 6, 7, 8, 9, 10), and the S2 subunit that promotes the viral-to-host cell membrane fusion event needed for viral infection (1, 7, 11, 12, 13, 14, 15, 16, 17). Here, ACE2 is linked to viral infectious disease.