An agent that experimentally inhibits the proliferation and viability of infectious viruses. In the domain of herbal medicines, some plants will slow or inhibit the adsorption or random initial attachment of viruses, extend the lifespan of infected target cells, or speed up several aspects of immunity, including complement, antibody, and phagocytosis responses. Herbal antivirals work best on respiratory viruses such as influenza, adenoviruses, rhinoviruses, and the enteric echoviruses. Touted as useful in the alphabet group of slow viruses (HIV, EBV, CMV, etc.), they really help to limit secondary concurrent respiratory infections that often accompany immunosuppression.