Complex sentence: After a period of prolonged stability--often several years--individuals infected with polio begin to experience new signs and symptoms, characterised by muscular atrophy (decreased muscle mass), weakness, pain and fatigue.
Split into simpler sentences: After a period of prolonged stability--often several years--previously infected individuals start to experience new signs and symptoms. The condition is characterised by muscular atrophy (decreased muscle mass), weakness, pain and fatigue.