<unk>  is the revival of material or behavior that had previously been stabilized, settled, or diminished. In medicine, it is usually defined as the recurrence of symptoms after a period of remission or quiescence, in which sense it can sometimes be synonymous with relapse. In a narrower sense, it can also be such a recurrence with higher severity than before the remission.
Recrudescence
Exacerbation
Convalescence
Remission 
Relapse
Spillover 
Seroconversion
Epizootic