Transmission of Norovirus Norovirus illness has been called by several names, including the stomach flu and winter vomiting disease. Noroviruses are important pathogens around the world. They are believed to be responsible for a large proportion of all gastroenteritis outbreaks worldwide. Approximately 21 million cases are estimated to occur each year in the United States. They are also the most common cause of foodborne illness in the United States. As much as half of all food-related outbreaks in the U.S. are believed to be caused by noroviruses. The virus needs to be taken in through the mouth to infect a person. It incubates in the gastrointestinal tract for 12 to 48 hours and most likely multiplies in the small intestine, causing tissue damage, which leads to the symptoms. The most common symptoms of norovirus infection are vomiting and diarrhea. Other symptoms include abdominal pain, nausea, fever, and headache. The illness usually resolves on its own without any long-term effects. Sometimes, people with certain underlying medical conditions can have more serious disease. The elderly are particularly susceptible. People acquire the virus through direct contact with people who are sick, as well as coming into contact with surfaces, foods, or water contaminated with the virus. The virus is shed in the millions to billions in stool and vomit, and the virus particles can also be aerosolized over several feet when a person vomits. It may take only 20 virus particles to make someone sick. Though shedding is highest during a person’s symptoms and just after symptoms resolve, people have been known to shed the virus for weeks after they feel better, potentially exposing others. It is important to note that not everyone is susceptible to any one strain of norovirus. A subset of the world’s population has a genetic makeup that prevents particular strains of the virus from being able to infect. It is suspected that not everyone who is infected with the virus shows symptoms. These viruses also have a high mutation rate and new strains emerge every few years. Coupled with the fact that norovirus infection does not create long-term immunity in most people, a person can be infected multiple times over the course of their life. Additionally, the viruses can persist on surfaces, and in food and water, and are not completely inactivated by most common disinfectants, sanitizers, and food processing and preservation methods. This complicates efforts to prevent and control the spread of the virus. People who study outbreaks, also called Epidemiologists, look at the distribution of cases as they develop over time. This helps to determine the original source of infection. For example, in this figure, a single vomiting incident followed by a contamination of a surface triggered a cascade of disease. In short, because the virus is shed in large amounts in vomit and feces, is persistent, and is resistant, it’s clear how a norovirus outbreak can result from one ill individual or contamination event.