Exile sickness

When a person is banished from Paragangia and their link is removed they are generally not well adapted to life outside the empire and many of the die from this condition fairly soon. when a link is removed some things can be done to protect a person from this decline but it will still effect them to some degree. 
Harris, although Sen'tran does all he can to protect him, suffers this to a degree when he goes to D'zeron, and it terrifies Jason because he remembers Jonathan as he is gradually declining into it. 
Jonathan having been linked at an older age is better able to endure it and also was protected by Sen'tran. 
see also Jonathan's Medical condition