Rotavirus is the leading cause of severe diarrhea in children under five. It is especially dangerous because vomiting and watery diarrhea can rapidly cause dehydration, the main immediate threat to young children with the infection.
Rotavirus belongs to the Reoviridae family and has a segmented double-stranded RNA genome. The name refers to its wheel-like appearance under electron microscopy. Infection commonly begins with fever and vomiting, followed by several days of watery diarrhea. Children can be infected more than once, but first infections are often the most severe.
The answer is sometimes complicated by geography and vaccination. Rotavirus vaccines have substantially reduced hospitalizations and deaths, so other pathogens may become relatively more prominent in highly vaccinated populations. Even so, global surveillance continues to identify rotavirus as a leading cause of severe pediatric diarrhea, particularly in settings without broad vaccine coverage. Norovirus, adenovirus, and several bacteria also cause serious diarrhea, but they are not the canonical answer to this question.