A highly contagious diarrheal illness is on the rise in parts of the Bay Area, with multiple cities recording increasing levels of the virus in wastewater in recent weeks. Rotavirus, a gastrointestinal virus, causes diarrhea and vomiting. Although usually mild, the disease can be dangerous for young children, hospitalizing 50,000 to 70,000 of them and causing between 20 and 60 deaths annually, according to the U.S. Centers for Disease Control and Prevention.