Is a 'Vomiting Virus' Sweeping the US? New Data Reveals Sharp Rise in Norovirus Cases Nationwide

A highly contagious virus known for causing sudden vomiting and diarrhoea is once again spreading across the United States, raising fresh concerns about public health as cases climb in unexpected regions.

What began as reports of sick hikers along Southern California's iconic Pacific Crest Trail has evolved into a broader national story. New surveillance data suggests norovirus activity is increasing across large parts of the country, with experts closely monitoring the situation despite insisting that current levels remain largely consistent with seasonal expectations.

Norovirus Cases Climb Across the United States

Recent data from WastewaterSCAN, a disease-monitoring programme led by Stanford University researchers, categorised norovirus activity as 'high' nationwide during late May. The surveillance system detected the virus in every wastewater sample collected during the month, with particularly elevated concentrations reported across the Northeast and Midwest. According to reports, the trend has continued to move upwards rather than declining as quickly as many would expect at this time of year.

The rise comes as health officials investigate an outbreak affecting hikers near Wrightwood in California's San Gabriel Mountains. More than two dozen hikers reportedly fell ill over a two-week period, with at least one person requiring an air rescue after developing severe symptoms.

Additional surveillance data from the US Centers for Disease Control and Prevention (CDC) has also shown a multi-week increase in positive norovirus tests following a seasonal decline that began in March. While positivity rates remain below winter peaks, the latest figures indicate that transmission remains active across multiple regions.

Why Experts Say the Surge Is Not Entirely Unexpected

Despite alarming headlines, public health experts caution against viewing the current rise as an unprecedented outbreak.

Amanda Bidwell, scientific programme manager for WastewaterSCAN, told USA Today that norovirus concentrations typically begin increasing during autumn, peak around early March and then gradually decline throughout spring. Current levels, she explained, are generally in line with what experts would expect for this stage of the year.

CDC data support that assessment. Through its NoroSTAT surveillance network, the agency tracked more than 1,000 norovirus outbreaks between August 2025 and April 2026. While significant, those figures remain within the historical range observed during previous seasons.

However, wastewater monitoring may provide a more complete picture than official outbreak counts because many infected individuals never seek medical treatment. Most people recover at home, meaning their cases are never formally recorded. Wastewater surveillance can therefore reveal hidden levels of community transmission that traditional reporting systems may miss.

A New Norovirus Strain May Be Fueling Infections

Another factor attracting attention is the emergence of a newer norovirus variant known as GII.17.

For more than a decade, the GII.4 strain dominated outbreaks across the United States. However, CDC researchers found that GII.17 accounted for approximately 75 per cent of norovirus outbreaks during the 2024–2025 season, overtaking GII.4 for the first time. Researchers believe lower population immunity to the newer strain may be contributing to increased transmission.

Experts emphasise that the symptoms caused by GII.17 are similar to those associated with older strains. However, because fewer people have previous exposure to the variant, it may spread more efficiently through communities.

What Is Norovirus and Why Is It Called the 'Vomiting Virus'?

Norovirus is the leading cause of acute gastroenteritis and foodborne illness in the United States. Often referred to as the 'winter vomiting bug' or 'stomach bug', the virus infects the stomach and intestines, causing sudden and often severe gastrointestinal symptoms and is notorious for its ability to spread rapidly through communities.

It spreads through contaminated food, water, surfaces or direct contact with infected individuals. What makes norovirus particularly difficult to contain is its remarkable infectiousness. Health experts note that only a tiny number of viral particles are needed to cause illness, while infected individuals can shed billions of particles during the course of an infection.

The illness is commonly known as the 'vomiting virus' because of its hallmark symptom: repeated and often sudden episodes of vomiting.

Symptoms, Timeline, and Prevention

Most people infected with norovirus develop symptoms within 12 to 48 hours of exposure. Common signs include nausea, vomiting, diarrhoea, stomach cramps and abdominal pain. Some individuals may also experience fever, headaches and body aches.

Although symptoms usually last only one to three days, dehydration remains a major concern, particularly among young children, older adults and people with weakened immune systems. Health officials continue to recommend frequent handwashing with soap and water, careful food preparation and staying home for at least 48 hours after symptoms have ended. These measures remain the most effective defences against one of the world's most contagious gastrointestinal viruses.