Three people have tested positive for norovirus amid a cluster of 28, the Pasadena Public Health Department said on Friday.

The U.S. is seeing a "quad-demic" as cases of COVID-19, flu, RSV and norovirus spread at the same time. Experts said this is what you need to know.

Norovirus, also known as the stomach bug, is spreading across the U.S. Here's what to know about the contagious virus and how best to prevent an infection.

The cases mark the fourth norovirus outbreak on Holland America ships since early December, following others on its Eurodam, Rotterdam and Zuiderdam vessels. Other companies, including Cunard Line and Princess Cruises also saw outbreaks in recent weeks.

You can also get norovirus by touching objects or surfaces that are contaminated and then putting your hands in your mouth without washing them first. Hand-washing with soap and water is key, as hand sanitizer will not do the trick in the case of norovirus, experts say.

Rates of norovirus in that CDC system have reached levels at or above last season's peak in all regions of the country. Norovirus test positivity rates look to be the worst in the Midwest, in a grouping of states spanning Kansas through Michigan.

CHICOPEE, Mass. (WWLP) – A new strain of Norovirus is impacting children and adults, and it’s spreading in our state. Respiratory illnesses continue to surge, that’s because it’s colder and more people are indoors allowing germs to spread, but it can be prevented.

No. Hand sanitizer is simply not as effective against norovirus as it at killing other pathogens due to the virus's firm shell, Dr. William Schaffner, professor of infectious diseases at Vanderbilt University Medical Center, told TODAY.com recently.

According to the Minnesota Department of Health, there have been 76 reported outbreaks in December which is higher than in previous years.

"It is too early to tell if this strain is associated with more severe norovirus disease but likely lower population immunity is the reason for the early surge of an otherwise seasonal increase of this virus," Jan Vinjé, head of the CDC's CaliciNet system, told CBS News in an email.

Ah, the classic signs of winter: Freezing temperatures, icy roads and empty tissue boxes. It's no secret that winter is a time when illness is prominent. Coughs, colds, and viruses are seemingly running rampant this time of year. This is thanks to the drier air and an influx of time spent indoors, according to the Mayo Clinic.