ACE2 and infection: Since the broad host range of SARS-CoV-2 is linked to the ubiquity of ACE2 and the high prevalence of SARS-CoV-2 in humans, repeated spillback infections (i.e., human hosts infecting animals with SARS-CoV-2 virus) can determine the establishment of new animal hosts from which secondary spillover can lead to human infections [26, 27].