We agreed to employ him for 2 years (or whatever the figure is) at a given rate of pay. There isn't the idea of "one month's notice" in that, it's a fixed length. I guess maybe in exceptional circumstances (like getting sent to prison) the contract is broken, but illness or no longer wanting the player is not a reason.
Think of the Geo Cronje situation. He came over here with a contract signed, all the doctors (on both sides) thought he had a minor injury. Turned out to be a play stopping injury, but Harlequins were obliged to continue paying him for the contract period. In the end Geo Cronje was too much of a gentleman to take the money for nothing and so the contract was dissolved by mutual agreement, but he could have insisted and Harlequins would have had to pay him out to get him off the books.