Flu (Influenza)

  • Medical Author:
    Charles Patrick Davis, MD, PhD

    Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.

  • Medical Editor: Melissa Conrad Stöppler, MD
    Melissa Conrad Stöppler, MD

    Melissa Conrad Stöppler, MD

    Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

Flu Slideshow: 10 Foods to Eat When You Have the Flu

Quick GuideFlu Pictures Slideshow: 10 Foods to Eat When You Have the Flu

Flu Pictures Slideshow: 10 Foods to Eat When You Have the Flu

What is the incubation period for the flu?

Incubation period for the flu, which means the time from exposure to the flu virus until initial symptoms develop, typically is one to four days with an average incubation period of two days.

How long is the flu contagious, and how long does the flu last?

The flu is typically contagious about 24-48 hours before symptoms appear (from about the last day of the incubation period) and in normal healthy adults is contagious for another five to seven days. Children are usually contagious for a little while longer (about seven to 10 days). Individuals with severe infections may be contagious as long as symptoms last (about seven to 14 days). In adults, flu symptoms usually last about five to seven days, but in children, the symptoms may last longer (about seven to 10 days). However, some symptoms such as weakness and fatigue may gradually wane over several weeks.

How is the flu (influenza) diagnosed?

The flu is presumptively diagnosed clinically by the patient's history of association with people known to have the disease and their symptoms listed above. Usually, a quick test (for example, nasopharyngeal swab sample) is done to see if the patient is infected with influenza A or B virus. Most of the tests can distinguish between A and B types. The test can be negative (no flu infection) or positive for types A or B. If it is positive for type A, the person could have a conventional flu strain or a potentially more aggressive strain such as H1N1. Most of the rapid tests are based on PCR technology that identifies the genetic material of the virus. Some rapid influenza diagnostic tests (RIDTs) can screen for influenza in about 10-30 minutes.

Swine flu (H1N1) and other influenza strains like bird flu or H3N2 are definitively diagnosed by identifying the particular surface proteins or genetic material associated with the virus strain. In general, this testing is done in a specialized laboratory. However, doctors' offices are able to send specimens to specialized laboratories if necessary.

Medically Reviewed by a Doctor on 4/28/2016

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