relapse

Concise Oxford English Dictionary © 2008 Oxford University Press:
relapse /rɪˈlaps/

▶verb

• 1 (of a sick or injured person) deteriorate after a period of improvement.

• 2 (relapse into) return to (a worse or less active state).

▶noun also /ˈriː-/ a deterioration in health after a temporary improvement.

– derivatives
relapser noun.

– origin ME: from L. relaps-, relabi ‘slip back’.