The invention provides a microbicidal composition comprising at least one siRNA. The siRNA is an RNA duplex made of one or two molecules. A portion of the siRNA is identical to a target sequence in an essential gene of a virus. The virus may be a herpesvirus, for example, HSV-1 or HSV-2. Preferably, the herpesvirus is HSV-2. The microbicidal composition further comprises a pharmaceutically acceptable carrier. Also included in the invention are methods to prevent and treat viral infections by administration of the microbicidal composition. Preferably, the microbicidal composition is administered transmucosally.
