A rapidly spreading stomach virus is making a comeback in California, with recent wastewater analyses indicating increases in several cities.

The WastewaterSCAN project has identified “high concentrations” of rotavirus in areas such as Marin, Redwood City, San Jose, and Santa Cruz. Meanwhile, “moderate concentrations” have been found in Sacramento, Davis, San Francisco, Sunnyvale, Fremont, Vallejo, and Novato.

This highly contagious virus is known to cause severe watery diarrhea and vomiting, especially in infants and young children, according to the U.S. Centers for Disease Control and Prevention.

Older children and adults with compromised immune systems are also at risk of contracting the virus.

Transmission occurs through tiny particles from infected fecal matter, which can spread via direct contact or through contaminated surfaces.

Before the introduction of a vaccine in 2006, rotavirus was the primary cause of severe diarrhea among young children in the United States, with up to 2.7 million cases reported annually, as per the California Department of Public Health.

Today, the virus still causes between 20 and 40 deaths each year, roughly 20 fewer than before vaccines became widely available.

The surge extends beyond California.

Federal data shows rising levels nationwide since mid-December, with high activity in every region except the Midwest.

The Bay Area is a major hotspot, while elevated levels have also been reported in New Jersey, Connecticut and along the Northeast coast.

The increase comes as federal vaccine guidance shifts. The administration now advises parents to decide with their doctors whether children should receive the rotavirus vaccine, moving away from previous CDC recommendations that all children be vaccinated.

Health Secretary Robert F. Kennedy Jr. said the change aims to align U.S. policy with international standards and emphasize informed consent.

The American Academy of Pediatrics noted that Denmark, cited as a model, does not vaccinate against rotavirus.

A court temporarily blocked the updated guidance last month, adding uncertainty around future vaccination rates.