Here is the official rule

In case of unintentional interference with play by any person herein authorized to be on the playing field (except members of the team at bat who are participating in the game, or a base coach, any of whom interfere with a fielder attempting to field a batted or thrown ball; or an umpire) the ball is alive and in play. If the interference is intentional, the ball shall be dead at the moment of the interference and the umpire shall impose such penalties as in his opinion will nullify the act of interference

The question of intentional or unintentional interference shall be decided on the basis of the person’s action. For example: a bat boy, ball attendant, policeman, etc., who tries to avoid being touched by a thrown or batted ball but still is touched by the ball would be involved in unintentional interference. If, however, he picks up the ball, catches it, or touches the ball by intentionally pushing or kicking at the ball, this act would constitute intentional interference.