Complex sentence: Malnutrition was previously seen as something that exacerbates the problems of diseases such as measles, pneumonia and diarrhea, but malnutrition actually causes diseases, and can be fatal in its own right.
Split into simpler sentences: Malnutrition was previously seen as something that exacerbates the problems of diseases as measles, pneumonia and diarrhea. But malnutrition actually causes diseases and can be fatal in its own right.