During the 2024–25 season, the GII.17 norovirus strain surpassed GII.4 as the predominant strain in the U.S., causing about 75% of outbreaks. The more mutated GII.17 strain partially evades prior immunity because people have had less exposure to it, allowing it to spread more widely despite causing symptoms similar to those of GII.4.

Wastewater surveillance shows norovirus levels are at 'high' levels across much of the country, with rising rates in the Northeast. Amanda Bidwell, scientific program manager at WastewaterSCAN, stated, 'At the national level, norovirus is still in the HIGH category due to high concentrations over the last 21 days.' Wastewater data also highlight a norovirus outbreak in the San Francisco Bay Area, and hikers in Southern California have been infected in recent weeks, according to the Pacific Crest Trail Association.

Norovirus is often called the 'winter vomiting disease,' but outbreaks can continue through late spring, with peak activity typically occurring from November through May in the Northern Hemisphere. National norovirus levels are not 'unusually high' compared with prior seasons, according to the Centers for Disease Control and Prevention. From August 1 to May 7, the CDC’s NoroSTAT program recorded 1,194 norovirus outbreaks, compared with 2,534 during the same period the previous year. CDC tracking is based on confirmed cases from state agencies and may not capture many illnesses, as most people recover at home without seeking medical care.

Dr. Linda Yancey, infectious disease specialist at Memorial Hermann, stated, 'The newer variant isn’t more contagious in and of itself. It can spread more easily because fewer people have partial immunity to it, so they will get sick and spread the virus.' She added, 'There really isn’t anything unusual about this one in California. They just got unlucky.' Bidwell noted that wastewater monitoring is especially valuable for tracking norovirus because 'there is not a lot of clinical data to describe outbreaks.'

Norovirus spreads easily between people, with each infected individual potentially transmitting the virus to up to seven others. Transmission occurs through direct contact with a sick person, touching contaminated surfaces, consuming contaminated food—especially ready-to-eat cold items like salads—or drinking inadequately treated water. The virus is the most common cause of food poisoning in the U.S. Most healthy adults recover within several days, though infected individuals can remain contagious for up to two weeks after symptoms subside. There is no specific treatment, and antibiotics are ineffective; experts recommend oral rehydration fluids over sports drinks for symptom management.