Disclosed is a highly safe antiviral agent that exhibits an effect upon preventative ingestion. Administration of said antiviral agent produces an effect against any kind of virus. A neutral polysaccharide produced by Lactobacillus delbrueckii subsp. bulgaricus OLL1073R-1 heightens the increase in NK cell activity that occurs in response to IFN-α, thereby enhancing natural immunity and providing a preventative effect that can start before virus infection and does not have viral specificity like vaccines do.
