The invention relates to pharmacology and may be used for producing medicinal agents for treating and preventing viral diseases caused by the herpes, influenza and hepatitis B and C viruses, and also virus-induced immunodeficiencies. A biologically active substance having a polypharmacological effect is made from the green portions and spikelets of cereal plants of the family Gramineae, genus Calamagrostis Adans and/or genus Deschampsia Beauv, and contains flavonoids, specifically aglycones of the flavonoids tricin, apigenin, luteolin, quercetin and rhamnazin, and/or flavonoid glycosides of tricin, apigenin, luteolin, quercetin, and rhamnazin, and excipients, and has the following composition by mass percent: tricin flavonoid aglycone and/or tricin flavonoid glycosides: 0.016-2.062%； apigenin flavonoid aglycone and/or apigenin flavonoid glycosides: 0.010-1.393%； luteolin flavonoid aglycone and/or luteolin flavonoid glycosides: 0.01-4.979%； quercetin flavonoid aglycone and/or quercetin flavonoid glycosides: 0.001-0.771%； rhamnazin flavonoid aglycone and/or rhamnazin flavonoid glycosides: 0.104-0.203%； excipients: 99.867-90.592%. In this way, the biologically active substance is perfected by determining the specific compositions of the active ingredients thereof, and the physical, chemical and biological characteristics thereof, resulting in the invention of an optimal composition (Fig. 1) for achieving an anti-viral effect with regard to specific viruses, and dosages when creating medicinal forms. In addition, it has been determined that the biologically active substance is an inducer of a type-y endogenous interferon, displays an apoptosis-modulating effect, has antioxidant properties and enhances cell resistance to free radical stress. The anti-viral effect with regard to specific viruses has been established to be an anti-viral effect with regard to the type 2 herpes simplex virus, the influenza virus, and the bovine viral diarrhea virus (hepatitis C virus).La inv
