Norovirus: the leading cause of acute gastroenteritis outbreaks in many countries Norovirus can spread through a very small amount of viral particles and is responsible for hundreds of millions of gastrointestinal illness cases worldwide each year. The World Health Organization has estimated that norovirus causes about 685 million cases of acute gastroenteritis annually , including about 200 million cases among children under five . The U.S. Centers for Disease Control and Prevention (CDC) has stated that, in the United States, norovirus causes about 19 million to 21 million illnesses each year , resulting in approximately 109,000 hospitalizations and 900 deaths . As of 2026, norovirus remains a major public health concern because it spreads quickly in homes, hospitals, care facilities, cruise ships, schools, childcare centers and food-service settings. It is not related to influenza, despite being widely called “stomach flu.” Norovirus is a group of viruses that inflame the stomach ...