Winter flu season – plus three other viruses

It’s winter. It’s winter 2025. The COVID pandemic is past, the Bird Flu is still quite rare in humans, but it is flu season.

But not just normal flu season, nationally hospitals and physicians are describing the 2025 virus season as a “Quad-demic.”

Four separate viruses are spreading and becoming more prevalent. First there is the annual influenza virus. A COVID variant is still prevalent across the country as is the respiratory syncytial virus. RSV is a common respiratory virus that infects the nose, throat, and lungs. RSV symptoms make it difficult to distinguish it from the common cold or other respiratory viruses (such as the flu or COVID-19). These three are all respiratory illnesses.

Norovirus is the fourth spreading virus, it attacks the gastrointestinal system causing diarrhea and vomiting, according to the Centers for Disease Control.

It’s a very active virus season, a quad-demic.

As of Jan. 17, the CDC reported, “. . . the amount of acute respiratory illness causing people to seek healthcare is at a high level.” All three respiratory virus illnesses are increasing across the U.S.

In California, emergency room visits for influenza, COVID-19 and RSV are increasing. Overall statewide, only flu was considered high. COVID-19 was at a low level and RSV is moderate, according to the California Department of Public Health.

“Nationally, influenza test positivity increased to 18.8%. COVID-19 test positivity remained stable at 6.6% and RSV test positivity decreased to 8.9%,” the CDC reported.

In California, the CDPH reported influenza test results were nearly 23%. The COVID test results for the second week of January were significantly lower than this time in the past two years.

At the end of the second week of January, the Riverside County’s Public Health Department reported influenza cases were moderate. However, emergency room visits were higher than one year ago but recently began to decline.

COVID-19 cases are significantly less than in 2024. COVID and RSV emergency room visits are both lower than a year ago.

For the next two weeks, emergency room visits for influenza will remain high to very high, the CDC forecast.

These viral illnesses are much more prevalent in winter for several reasons. First, the cold weather causes more frequent gatherings of people indoors. Secondly, winter’s cold and dry weather are more conducive to virus spread and often it is a time of more susceptible immune systems.

Hand washing is an effective means for prevention. Extra precautions include wearing a mask or reducing direct interactions until healthy.

In California several counties such as San Francisco, Alameda, San Mateo, Contra Costa, Napa, and Berkeley are requiring masks in certain healthcare and nursing facilities.

Competing with the viral “quad-demic” is a bacterial infection — whooping cough, which is also on the increase. Whooping cough, also known as pertussis, is highly contagious and can be spread though coughing and can cause severe illness, especially in infants. CDPH urges all Californians, especially those who are pregnant and those who provide care to infants, to be up to date on their Tdap (tetanus, diphtheria, pertussis) vaccine to help prevent whooping cough. 

More than 2,000 cases (62 were infants) of whooping cough were reported statewide between January and October 2024. During the same time range in 2023, there were less than 400 whooping cough cases reported in the state.