At Riverside Community Hospital, for example, the number of people checking in to the ER with flu-like symptoms jumped by 50 percent in the past week.
Health officials are saying the flu outbreak is the worst they've seen in 15 years.
"We haven't declared it an epidemic, but definitely we have increased activity, said Barbara Cole with Riverside County Disease Control.
Doctors say many of the patients don't need to actually come to the hospital. In most cases, they can just stay home to recover.
"The flu is one of those things that can take up to two weeks to get better from and unfortunately does not respond to regular antibiotics," said Dr. Steven Kim with Riverside Community Hospital.
Doctors say one sign that you should go to the hospital is if you are having a lot of trouble breathing, as that could indicate it has progressed into the lungs, a dangerous complication.
So why are they seeing so many case of the flu this year? Perhaps one reason is that people aren't getting flu shots this year, because they're apparently 30 to 40 percent less effective.
"Is it that fewer people got vaccinated? Is it a more virulent strain? At this point we don't have a definitive 'this is why it's up,"" Cole said.
Another reason is that the most common strain of flu this year is an A strain, which is more powerful than other varieties that could be common in a given season.
In Burbank, Providence St. Joseph Medical Center is seeing about 280 patients a day - well above the usual 180 patients a day, said Dr. Stephen Kishineff.
That means the ER is at "Code Black" - or at its full capacity.
"The flu itself is more virulent this year," Kishineff said. "It's causing more symptoms, higher fevers, more body aches. It's causing some of the older people to pass out."
Older people may also experience mild heart attacks from the heart enzyme elevations caused by this flu strain, he said.
The flu season could last well into March. People, especially those in vulnerable populations, are advised to be cautious when going out in public, be sure to wash their hands and get plenty of rest.