Health experts say the flu has already claimed the lives of two people in Kings County, a Fresno County woman, and two adults in Merced County.
Those Valley flu deaths are part of 28 being investigated by the state department of health, which has already confirmed seven flu-related deaths. Doctors say most of cases they're seeing involve the H1N1 strain.
The H1N1 strain caused a worldwide pandemic in 2009. 607 people in California alone died of the swine flu. The virus has now made a dangerous return. Doctors say most flu patients are infected with H1N1.
Dr. Gil Chavez said, "One thing we're noticing this particular year is that people get sick pretty quickly and people having reported to actually go from being healthy to having the flu to being in the hospital within two days."
This year's flu vaccine protects against the H1N1 strain. Doctors urge anyone over six months old to get vaccinated, especially pregnant women and those with chronic medical conditions.
Dr. Dee Lacy is an infectious disease physician with Kaiser Permanente. She says seniors aren't the only ones who need flu shots.
Dr. Dee Lacey explained, "The data shows it is younger people and young adults or middle age adults having more trouble or getting into more trouble which may also be that traditionally children and adults are not vaccinated against influenza."
The flu season typically doesn't peak until kids return from Christmas vacation, which is why it's so important to use hand sanitizer and regularly wash your hands.
"Definitely schools are places where we'll see a lot of infection and transmission," said Dr. Dee Lacey. "The incubation is only one to three days."
Dr. Gil Chavez from the state department of public health also mentioned it did not appear the H1N1 strain has changed in any way since the pandemic of 2009.