PROBLEM TO BE SOLVED: To provide an infection preventive agent which has excellent efficacy of preventing influenza viruses infection, and is useful for preventing and treating influenza, and to provide foods and drinks and feed containing the infection preventive agent.SOLUTION: An infection ameliorative agent is characterized by containing Lactadherin and/or a decomposition product obtained by treating Lactadherin with protease, as an active ingredient. The infection preventive agent of the invention can be manufactured simply and economically owing to using mammalian milk as a raw material, and can be ingested daily and safely.