To provide safe and easy means for preventing or treating diseases of birds and mammals, in particular, livestock. In particular, to provide means for preventing or treating an infectious disease caused by a Gram-positive bacterium. In addition, to improve fermentation in the rumen of a ruminant animal, to contribute to suppression of the generation of greenhouse gas, and to increase the feed efficiency. Mannosylerythritol lipids (MEL) and/or rhamnolipids are given to birds or mammals.