In developing countries, diarrhea, which is a symptom of gastrointestinal infection, is one of the biggest health concerns. Diarrhea kills 2.5 million children each year, but little data about diarrhea and climate exists. Karen Levy, an environmental epidemiologist at Emory University in Atlanta, found that rotavirus, which causes diarrhea, becomes more active in tropical regions when the climate is cooler and drier.