Antiviral drug. Abstract illustration of the antiviral drug acyclovir attacking a herpesvirus. Acyclovir acts by specifically interfering with an enzyme in the virus called DNA polymerase, thus preventing viral DNA (deoxyribonucleic acid) replication. The viral DNA is enclosed in a 20- sided icosahedron (yellow and red) made of hexagonal protein subunits. This protein coat, called the capsid, is itself enclosed in a phospholipid membrane envelope, represented here by a sphere. The projections on the envelope are glycoproteins that enable the virus to bind to receptors on the membranes of target host cells.