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What is Influenza?

Influenza (also called flu) is a contagious virus that can infect the nose, throat and sometimes the lungs. It can cause mild to severe illness, and sometimes even death. Although influenza affects all ages, it is most commonly spread among young children, and from them infects their household and other contacts, affecting the whole community. It can be especially severe for those with certain chronic medical conditions (such as asthma, diabetes, or heart disease) or weakened immune systems, and for young children (<5 years of age), pregnant women, or people 65 years of age or older. Influenza viruses are constantly changing so people can get infected with influenza every year. The best way to prevent influenza is through annual vaccination. Find out where to get your flu shot and how to prevent flu this season.


Influenza Surveillance

Each influenza season, Orange County Health Care Agency partners with local hospitals, primary care providers, and universities to conduct influenza surveillance. Annual influenza epidemics follow a winter seasonal pattern in the United States with typical activity peaking during late December to February. Surveillance activities help to monitor and prepare for the impact of influenza on the healthcare system. Moreover, influenza surveillance in California is vitally important to monitor for importation of novel strains of influenza from other parts of the world.

Surveillance for influenza activity in Orange County includes monitoring of the following components year-round:

  • Influenza-like Illness: sentinel sites including pediatric and adult outpatient clinics, university student health clinics.
  • Laboratory: 26 acute care hospitals, sentinel ILI sites, electronic laboratory reporting.
  • Hospital admissions/Deaths: acute care hospitals, vital records.

The following types of influenza infections are reportable in Orange County:

  • Laboratory confirmed cases 0-64 years of age who are admitted to an intensive care unit (ICU); OR
  • Laboratory confirmed cases 0-64 years of age who have died; OR
  • Novel strains of influenza (including suspect human cases of avian influenza, variant influenza, or pandemic influenza); OR
  • Cases associated with an outbreak.

Any suspected novel influenza cases or outbreaks of influenza or influenza-like illness should be reported immediately by phone (714-834-8180) to OCHCA Communicable Disease Control Division. Healthcare providers, hospitals and laboratories may be asked to submit additional reports or specimens as part of influenza surveillance or enhanced surveillance for novel strains.


Healthcare Professionals


Influenza-like Illness (ILI) Sentinel Provider Program

We are actively recruiting physicians and other health care providers to participate as sentinel providers for influenza surveillance. For more information about becoming a sentinel provider, please contact us


Additional Information


Last Updated:  12/7/2023