Epidemiology Unit

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What is influenza?

Influenza, commonly referred to as the flu, is a respiratory illness caused by influenza viruses. There are two main types of influenza virus, types A and B. While influenza A viruses are found in many different animals, including birds and swine, humans are considered the primary reservoir for the influenza viruses responsible for seasonal influenza epidemics: influenza A(H1N1), influenza A(H3N2), and influenza B. In the Northern Hemisphere, seasonal influenza epidemics (the flu season) occur annually between October and May, usually peaking in January-February. The severity of each influenza season is related to many factors, including the viral characteristics of circulating strains (such as antigenic type, virulence, and transmissibility), natural or vaccine-related immunity in the population, and the health of the population. Influenza viruses mutate constantly and can combine with other strains in an infected person or animal to create slightly or even drastically new (novel) strains each season. Pandemics occur when a novel strain of influenza emerges that is efficiently transmitted in a sustained way to a susceptible global population. For more information on Seasonal Influenza, please click the link below.


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Respiratory Virus Surveillance Report

The County of San Diego performs influenza surveillance year-round. 

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Summary reports of past influenza seasons can be found on the Epidemiology Data and Reports page.


Information for Health Professionals

Physicians and laboratories are encouraged to report all influenza detections (e.g., rapid test, PCR) to the San Diego County Epidemiology Unit. Hospital laboratories and clinical offices not yet connected to electronic laboratory reporting (ELR) are requested to report via CMR form, Influenza Case Report form, or by simply sending a copy of the laboratory test result to the Epidemiology Unit by fax to (858) 715-6458.

Pediatric (ages 0-17 years) deaths due to influenza are legally reportable to the San Diego County Epidemiology Unit and should be reported via CMR form or Influenza Case Report form.

Influenza outbreaks are legally reportable to the San Diego County Epidemiology Unit and should be promptly reported by telephone at (619) 692-8499 Monday-Friday 8 AM-5 PM, or (858) 565-5255 on evenings, weekends, and County-observed holidays in the following circumstances:

  • Outbreaks in institutions and other residential facilities (e.g., long term care facilities, homes for the developmentally disabled or chronically ill, sleepover camps, prisons) with at least one case of laboratory confirmed influenza in the setting of a cluster (≥2 cases) of influenza-like illness (ILI) within a 72-hour period. Influenza-like illness (ILI) is defined as fever (≥100°F or 37.8°C) and a cough and/or a sore throat, in the absence of a known cause other than influenza.

The following guidance documents and resources are available to assist with preventing, managing, and controlling influenza outbreaks in long-term care facilities and similar settings.

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For more information, contact the Epidemiology Unit at (619) 692-8499 or send us an e-mail.