An antiviral agent is provided, having a phosphorodiamidate morpholino oligomer with an antisense sequence to a portion of a genome of a strain of Zika virus (ZIKV). The antiviral agent finds many uses, such as in a pharmaceutical composition, a method of treating ZIKV-mediated disease, a method of preventing ZIKV-mediated disease, a method of reducing or preventing the replication of ZIKV in a host cell, a method of controlling the spread of ZIKV in donated tissue, a treated tissue sample, and in the manufacture of a medicament for the treatment or prevention or ZIKV-mediated disease.
