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

This fall saw an unusually high number of cases of “walking” or mycoplasma pneumonia, a mild pneumonia that typically causes a severe cough but less fatigue than more serious forms of the disease.

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.