Antiviral compositions including Lycoris squamigera extracts are described as useful for preventing or treating diseases caused by influenza virus infection of humans and other mammalian and avian subjects (e.g., pigs, horses, birds, and the like). Lycoris squamigera extracts in such use exhibit low toxicity in normal cell environments, and excellent antiviral effects. Compositions and anti-viral agents for influenza virus that include Lycoris squamigera extracts are effectively used in foods and pharmaceutical products for preventing and treating influenza virus diseases.