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What is influenza?

Influenza, otherwise known as "the flu", is an infectious respiratory illness that is caused by an influenza virus. This virus is spread from person-to-person through direct contact with respiratory droplets from an infected person (usually from a cough or sneeze). Influenza is different from a "common cold".

What are the symptoms?

"Flu like symptoms" usually have sudden onsets and may include high fever, headaches, extreme fatigue, dry cough, sore throat, runny or stuffy nose, muscle aches, and stomach symptoms (nausea, vomiting, and diarrhea). These symptoms may be mild to severe and rarely, influenza can cause death. Most people recover from the flu quickly without any complications. Young children, pregnant women, people with chronic diseases, and people who are 65 years and older should take caution during the "flu season".

When is the flu season?

Influenza can occur throughout the year, but seasonal influenza is usually from October through mid-May of the following year. The season generally peaks from December to March.


How is influenza diagnosed?

Influenza is usually diagnosed based on symptoms and treatment is not necessarily dependent upon laboratory testing. To confirm the influenza diagnosis, laboratory testing is needed. Laboratory testing can confirm the presence or absence of influenza and determine the type of the influenza virus.

Delaware uses the following criteria to determine influenza case status:*

Confirmed influenza:

  • The viral culture is positive for influenza.
  • The influenza Polymerase Chain Reaction (PCR) test is positive.

Unconfirmed influenza:

  • Rapid influenza test positive without performing confirmatory test. (This is what they do in the doctors' offices and in the hospitals.)
  • Acute serological (blood) testing for the Influenza A and/or B antibodies. Unless a convalescent (after the patient has recovered) blood test is performed, the acute (while patient is actively ill) blood test alone is not confirmatory. There must be a 4-fold increase in the titer between the acute and convalescent blood tests. Most doctors do not order the convalescent blood test since the patient has generally recovered.

Common types of influenza:

There are two main types of influenza viruses that cause seasonal epidemics: Type A and Type B. Type A usually causes more severe symptoms. Type B causes similar symptoms but is generally less severe.

Ways to prevent influenza:

  • Get vaccinated. Receive vaccination before the flu season, generally in the fall. However, you can receive vaccination anytime during the season. It takes about two weeks for a person to develop antibodies which provide protection against the influenza virus.
  • Avoid close contact. Avoid close contact with people who are sick. When you are sick, keep your distance from others to protect them from getting sick, too.
  • Stay home when you are sick. If possible, stay home from work, school or errands when you are sick. You will help prevent others from catching your illness.
  • Cover your mouth and nose when you are sick. Cover your mouth and nose with a tissue when coughing or sneezing.
  • Wash your hands thoroughly. After coughing or sneezing, wash your hands with soap and water or clean with an alcohol-based hand cleaner. Washing your hands frequently is the single best defense against the spread of germs.
  • Avoid touching your eyes, nose, or mouth. Germs are often spread when you touch something that is contaminated with germs and then you touch your eyes, nose, or mouth.
  • Practice other good health habits. Get plenty of sleep, be physically active, manage your stress, drink plenty of fluids, and eat nutritious food.

*Criteria to determine influenza case status may differ state to state.






Link below to DelawareInfluenza Weekly Activity Level and Reports.
Flu Reports


Influenza FAQ Sheet

[ Influenza FAQ Sheet -
Text Version


Public Health
Flu Clinic


Immunization Program -


Centers for Disease
Control and Prevention
Seasonal Flu
Information Page


Flu Activity
& Surveillance


Home Page


CDC 2009
H1N1 Flu (Swine Flu)
Information Page


Public Health


Office of Infectious Disease Epidemiology

24/7 Emergency Contact Number: 1-888-295-5156

Return to Infectious Disease Home Page

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