Highclere Castle, the grand house used as Downton Abbey, actually served as a convalescent home during the first world war. Due to the extraordinary number of injured soldiers that were streaming in from the Western Front, many country estates in England opened their doors when the hospitals soon became overrun. While some volunteered their home, such as Highclere Castle, others were pressed into service. It was thought that the fresh country air was the best for recuperation, plus these sprawling estates had the room to spare.
You can learn more about Highclere’s castle’s turn as a convalescent home here, and see a video about it here. There’s also a really an interesting article about the current Countess of Carnavon, who has written a book about the real Downton Abbey here.