An anti-influenza virus agent, an anti-RS virus agent, or an anti-immunodeficiency virus agent, which comprises components originated from plants and which has excellent infectivity-inhibitory effect and proliferation-inhibitory effect against influenza viruses, RS viruses and immunodeficiency viruses. The anti-influenza virus agent, anti-RS virus agent or anti-immunodeficiency virus agent of the present invention comprises an extract derived from, or dry powder of the leaves of a plant belonging to the family Betulaceae and/or a plant belonging to the family Meliaceae.