Sentence:  Although Crohn disease and ulcerative colitis are chronic, they are characterized by periods of remission that last for varying amounts of time, interrupted by <WSD>flare-ups</WSD>, when symptoms of the disease become more evident. 
Meaning of the word marked with <WSD> tags:  A sudden outbreak, outburst or eruption (originally of flame, but now used more generally of any violent activity or emotion).