Complex sentence: Persistent diarrhea is an episode that starts acutely but then lasts for 14 days or more; it is an important cause of morbidity and mortality in children under five years old in developing countries.
Split into simpler sentences: Persistent diarrhea is an episode that starts acutely but then lasts for 14 days or more. It is an important cause of morbidity and mortality in children under five years old in developing countries.