Norovirus cases have risen in the United States in recent weeks, according to data from the Centers for Disease Control and Prevention. 

Nearly 14% of tests for the illness came back positive during the week that ended on Nov. 15, according to CDC data. Over 2,700 tests were conducted that week, according to the CDC, meaning that about 380 cases of norovirus were diagnosed in that period.  

That’s more than double the percentage of positive tests recorded about three months ago, when test positivity in the U.S. this year was at its lowest. The week that ended on Aug. 9 saw a 6.53% positivity rate of more than 4,240 tests conducted during that period, according to CDC data.

Test positivity is an imperfect metric because it can vary based on the number of tests conducted. The CDC’s data is based on reports from participating laboratories, hospitals and health departments. Test positivity rates can help provide general data about what diseases are circulating and when, the CDC said. 

Norovirus, sometimes known as the stomach bug or stomach flu, is the leading cause of vomiting and diarrhea in the United States, according to the CDC. It is also the leading cause of foodborne illness, the agency says on its website. Norovirus is extremely contagious and causes symptoms including nausea, belly pain, vomiting and diarrhea. Most people experience symptoms for one to three days. 

Millions of people are sickened with norovirus every year, the CDC says, and about 2,500 outbreaks of norovirus are reported in the U.S. each year. The illness results in over 100,000 hospitalizations and about 900 deaths per year. Adults 65 and older are the most likely to die from norovirus, the CDC said. Young children and those with compromised immune systems are also at a higher risk for severe illness or death. 

Outbreaks of norovirus are most common between November and April, the CDC says. Last winter, the U.S. saw a major surge of norovirus, with a more than 25% test positivity rate recorded in January 2025. 

More recently, earlier this month, a potential outbreak prompted a Michigan middle school to close for a day for deep cleaning after dozens of students reported feeling ill. Nearly 100 cruise ship passengers were sickened by norovirus in October. 

Washing hands frequently, safely handling and preparing food, and regularly cleaning surfaces can help prevent norovirus from spreading, the CDC says. The disease is also contagious after symptoms end, so people should continue these precautions for at least two days after they begin to feel better, the CDC says.