Influenza A, COVID-19 and norovirus (stomach flu) are on the rise, according to the Centers for Disease Control and Prevention. But there's a fourth "bug" the public should be aware of: respiratory syncytial virus. RSV causes cold-like symptoms and affects the breathing passages and lungs.

Viral sicknesses have risen in early 2025 amid the ongoing “quad-demic” in the U.S., referring to three respiratory viruses and one gastrointestinal disease, according to the Centers for Disease Control and Prevention (CDC) data.

After the holiday season and an uptick in travel, California faces a rise in respiratory illnesses, prompting health experts to talk of a “quad-demic.” The term refers to the simultaneous spread of four major viruses: COVID-19,