Influenza - What is the Flu?

2014-2015 Flu Season

Purple flu virus particles
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Influenza, or the flu, has been around a long time.

There are historical reports of flu or flu-link epidemics since as early as 430 B.C. In fact, an early plague in Athens is now thought to have been caused by the flu.

A more recent report of the flu came from an epidemic in Italy in the 14th century and helped the flu got its modern name. That epidemic was thought to be caused by an "influence of the stars" or "influenza coeli."

It wasn't until 1931 that the influenza A virus was actually identified, though, first in pigs and then a few years later in humans.

What Is the Flu?

The flu is an infection that is caused by the flu virus.

That sounds like a simple enough explanation, but things get a lot more complicated when you start talking about the actual flu virus.

For one thing, there isn't just one flu virus. There are three main types of flu, types A, B, and C. And there are different subtypes of type A influenza that cause flu in people, including various combinations of the subtypes H1, H2, and H3 with N1, and N2.

All of these different strains of flu and the ability of the flu virus to frequently change is the reason that people don't become immune to the flu like they do after getting chicken pox or the measles. It is also one of the main reasons that you have to get a flu vaccine each year to protect yourself from the flu.

Some other important things to know about the flu include that:

  • type A flu can infect humans and many other animals, including birds, pigs, and ferrets, etc.
  • type B flu is usually milder than type A flu and only affects humans
  • type C flu is rare and does not cause flu epidemics

In addition to the virus type and subtype, flu virus strains are typically named after where they originated, a strain number, and when they were first isolated.

For example, one of the strains in this year's flu vaccine is named "A/California/7/2009 (H1N1)-like."

Flu Pandemics

Although the flu virus changes almost every year and causes a lot of people to get sick, it is only when there is a major change that we will see a worldwide pandemic. Called an antigenic shift, when this happens, the flu virus changes so much that many more people get sick than would in a regular flu season.

Flu pandemics are known to have happened in:

  • 1580 - likely the first true flu pandemic
  • 1889 - H3N2 - Russian flu, which likely spread around the world because of railroad travel
  • 1918 - H1N1 - Spanish flu, also known as The Great Pandemic, as it is estimated that over 50 million people died, including about 675,000 in the United States.
  • 1957 - H2N2 - Asian flu that killed about 70,000 people in the United States
  • 1968 - H3N2 - Hong Kong flu, which was considered a mild pandemic, with about 33,800 deaths in the United States
  • 2009 - H1N1 - Swine flu, affected up to 89 million people in the United States, with up to 18,300 deaths

    Other pandemic flu "threats" that caused concern, but didn't actually spread worldwide, occurred in 1976 (swine flu), 1977 (Russian flu), and 1997 (bird flu).

    History of Flu Vaccines

    Even with occasional delays or shortages, the availability of flu vaccines is something that many people take for granted these days.

    It wasn't until 1943 that the first flu vaccine was developed, though, first tested and used on soldiers in World War II. It took until 1963 for a yearly flu vaccine to be routinely recommended for high-risk groups, including people with chronic disorders, immune system problems, and the elderly.

    A few years earlier, in 1961, Dr. Luther Terry, the Surgeon General of the United States, did urge vaccination for people in high-risk groups and vaccination of healthy children and young adults between the ages of 5 and 25 years. This was in response to his prediction of a bad flu season. Many doctors didn't agree, though, including the American Academy of Pediatrics, which issued a statement against widespread immunization against influenza. By the end of that year's flu season, there were reports of a "larger number of influenza cases than had been expected," and epidemic influenza in 36 states.

    It wouldn't be until the 2002-03 flu season before there were recommendations for healthy children to get a flu shot again, starting with infants and toddlers between the ages of 6 and 23 months. Flu vaccine recommendations for healthy children continued to expand until we got to the point that all children and teens were supposed to get a flu vaccine for the 2008-09 flu season.

    A recommendation for universal flu vaccination (all children over six months and adults should get a flu vaccine) was instituted the next year.

    In addition to the flu shot, other flu vaccine options now include FluMist (2004), the nasal spray flu vaccine, Fluzone Intradermal (2011), and Fluzone High-Dose (2011).

    What You Need To Know about the Flu

    In addition to the fact that the best way to prevent the flu is by getting a yearly flu vaccine, it can be helpful to know that:

    • The incubation period for flu, the time from when you are exposed to the flu virus (usually when a sick person coughs or sneezes near you) to when you will develop flu symptoms, is about one to four days.
    • Classic flu symptoms typically include the sudden onset of a fever from about 101 to 102°F, muscle aches (myalgia), runny nose, nonproductive cough, sore throat, and a headache.
    • Flu season usually peaks sometime between December and March in most years.
    • Tamiflu (oseltamivir) and Relenza (zanamivir) are two anti-viral medications that can be used to treat and prevent the flu. Two older flu medications, amantadine, and rimantadine, are no longer used because of problems with resistance.
    • Children, including teens, should not be given aspirin if they have the flu because of an association with Reye syndrome. Use an age-appropriate dose of acetaminophen or ibuprofen instead.

    It is also important to note that 107 children died of the flu last year, in what was probably a mild-moderate flu season.


    Mamelund, S-E. Historical Influenza. International Encyclopedia of Public Health. 2008.

    The New York Times. Influenza Declining But Health Service Reports 2 Outbreaks in Wyoming. March 10, 1962.

    The Pink Book. Epidemiology and Prevention of Vaccine-Preventable Diseases. 12th Edition (April 2011).

    U.S. Department of Health & Human Services. History of Flu Pandemics.

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