When I was a young girl in Pakistan, my mother would remind me daily to only drink boiled water. We almost lost my sister to severe diarrhea and my mother was determined to make sure this didn't happen to our family again. Back then, I didn't fully understand her. It wasn't until years later, when I became a pediatrician and a child health researcher, that I realized how deadly watery stools can be. Unlike for adults, the rapid loss of liquids caused by severe diarrhea can bring children and babies to the brink of death in a matter of hours.