旧病复发

1. have a recurrence of an old illness; have an attack of a recurrent sickness; have a relapse

2. relapse into one's old bad habits; slip back into one's bad old ways

♦ 他发过誓不再偷了, 可不久又~, 走上了犯罪道路。 He swore that he would never steal again, but soon relapsed into a life of crime.