Antiviral protease inhibitors, including peptidyl aldehydes, peptidyl .alpha.-ketoamides, peptidyl bisulfite salts, and peptidyl heterocycles, are disclosed, along with related antiviral compounds, and methods of using the same to treat or prevent viral infection and disease. The compounds possess broad-spectrum activity against viruses that belong to the picornavirus-like supercluster, which include important human and animal pathogens including noroviruses, enteroviruses, poliovirus, foot-and-mouth disease virus, hepatitis A virus, human rhinovirus (cause of common cold), human coronavirus (another cause of common cold), transmissible gastroenteritis virus, murine hepatitis virus, feline infectious peritonitis virus, and severe acute respiratory syndrome coronavirus.