Although hundreds of viruses are known to cause human disease, antiviral therapies are approved for fewer than 10. Most approved antiviral drugs target viral enzymes, most commonly proteases and polymerases. Such direct acting antivirals (DAAs) have shown considerable success in the treatment of HIV and hepatitis C virus (HCV) infections. However, this approach does not scale easily and is limited particularly with respect to emerging and reemerging viruses against which no vaccines or antiviral therapies are approved.