<;P>;PROBLEM TO BE SOLVED: To provide an anti-PRRS virus agent capable of controlling a virus of viral diseases, especially PRRS virus as a cause of PRRS in pigs and to provide pig feed for controlling PRRS virus. <;P>;SOLUTION: The anti-PRRS virus (porcine reproductive and respiratory syndrome virus) agent contains at least one 6-12C medium-chain fatty acid and/or a salt thereof as active ingredients. <;P>;COPYRIGHT: (C)2010,JPO&INPIT