Paradoxically, although IFN-λ4 is an antiviral cytokine and a potent inducer of the JAK-STAT pathway, individuals carrying the IFNL4-ΔG allele have impaired ability to clear certain viral infections such as hepatitis C virus (HCV)16,18,19, cytomegalovirus20,21, coronavirus22,23, and possibly HIV24. Here, IFNL4 is linked to viral infectious disease.