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About Influenza
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What is the flu?

Influenza is also called the flu and is caused by the Influenza virus. This virus targets the respiratory tract and results in an infection that settles into the nose, sinuses, throat, bronchial tubes and possibly the lungs. Fever, headache, extreme fatigue and body aches are typical symptoms of influenza which are commonly associated with a dry cough, sore throat, and nasal congestion. Influenza symptoms typically come on suddenly and hit hard. Flu patients often describe the way they feel as, "It's like being hit by a truck."

The virus enters through the mouth or nose

A virus is an organism that is so small you can't see it without a strong microscope. Viral infections are not treatable with antibiotics (like penicillin). However, the viral infection from influenza can set the stage for a bacterial infection to develop. The influenza virus attacks the nose, sinuses, throat and lungs creating a large amount of mucous. This creates the ideal environment for bacteria to grow and develop into sinusitis, ear infections, bronchitis, and pneumonia. Once the bacterial infection takes hold, it can make the person quite ill, but fortunately the bacterial infection usually can be treated with antibiotics.

Every year in the United States, on average, 5% to 20% of the population gets the flu. More than 200,000 people are hospitalized from flu complications, and about 36,000 people die from the flu.(CDC) Those most likely to develop serious bacterial infections such as pneumonia and other life-threatening complications include young children, the elderly, people with chronic medical conditions, and those with weakened immune systems.

The usual flu season in the Northern Hemisphere is November through April, however, the flu can be caught at any time of the year. 

1918 Flu EpidemicThere are three types of influenza viruses: A, B, and C. Types A and B are the most severe. They change constantly and different strains of A and B viruses circulate around the world every year. The body's natural defenses cannot keep up with these changes, thus, a person who wants to prevent the flu should get a flu shot each year.

Types A and B are the most severe influenza viruses, but Type A is the most devastating of the three and is believed responsible for the global outbreaks of 1918, 1957 and 1968.

Influenza type A viruses can infect people, birds, pigs, horses, and other animals, but wild birds are the natural hosts for these viruses. Influenza type A viruses are divided into subtypes and named on the basis of two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA). For example, an “H7N2 virus” designates an influenza A subtype that has an HA 7 protein and an NA 2 protein. Similarly an “H5N1” virus has an HA 5 protein and an NA 1 protein. There are 16 known HA subtypes and 9 known NA subtypes. Many different combinations of HA and NA proteins are possible. Only some influenza A subtypes (i.e., H1N1, H1N2, and H3N2) are currently in general circulation among people. Other subtypes are found most commonly in other animal species. For example, H7N7 and H3N8 viruses cause illness in horses, and H3N8 also has recently been shown to cause illness in dogs.(CDC)

Swine Flu and Avian Flu viruses are both type A strains. These viruses typically do not usually infect humans so little natural immunity probably exists. This is one reason why both of these viruses are more dangerous than the seasonal flu.

It has been known for a long time that avian and swine strains of flu can spread to humans, with avian strains appearing to be more dangerous than swine strains. 

"Past experience has shown potential for new strains of influenza to spread extensively. On the other hand, there have been situations, such as the swine flu scare in 1976, which turned out to be false alarms", according to the CDC.

  • Avian Flu : Some types of Avian Flu viruses (H5N1, H7N7) are associated with severe and fatal disease. The viruses do not usually infect humans, however, confirmed cases of human infection have been reported since 1997, especially in Asia. There are three avian influenza A viruses (H5, H7, and H9) which theoretically can be partnered with any one of nine neuraminidase surface proteins; thus, there are potentially nine different forms of each subtype (e.g., H5N1, H5N2, H5N3, H5N9).

    Most human infections with avian influenza A viruses have occurred following direct contact with infected poultry. Human clinical illness from infection with avian influenza A viruses has ranged from eye infections (conjunctivitis) to severe respiratory disease (pneumonia) to death.

    • Pig farmSwine Flu (H1N1): The epidemic of 1918 is thought to be due to a swine flu virus. Since then only sporadic human infections with swine flu have occurred. Most commonly, these cases have occurred in persons with direct exposure to pigs (e.g. children near pigs at a fair or workers in the swine industry). In addition, there have been documented cases of one person spreading swine flu to others. For example, an outbreak of apparent swine flu infection in pigs in Wisconsin in 1988 resulted in multiple human infections, but no community outbreak resulted. In the past, there has been approximately one human swine influenza virus infection every one to two years in the U.S., but from December 2005 through February 2009, 12 cases of human infection with swine influenza have been reported to the CDC.

      • The symptoms of swine flu in people include fever >101.5, lethargy, coughing, runny nose, sore throat, body aches, headache, chills, fatigue, and lack of appetite. Diarrhea and vomiting have also been reported in those testing positive for the swine flu virus.

      • Symptoms last a week or longer.

      • The incubation period of swine flu is one to 7 days.

      • People may be contagious from one day before they develop symptoms to up to 7 days after they get sick.

      • There are two influenza antiviral medications that are currently effective against H1N1 flu. The drugs are oseltamivir (Tamiflu ®) and zanamivir (Relenza ®). They work best if given within 2 days of becoming ill, but may be given later if illness is severe or for those at a high risk for complications.

The type B virus widely circulates in humans and is generally responsible for the yearly outbreaks ferred to as "seasonal flu", for which we receive vaccinations.

Type C has been found in humans, pigs, and dogs and causes either a very mild illness, or has no symptoms at all. It does not cause epidemics and does not have the severe public health impact that influenza types A and B do.

How do you catch the flu?

A sneeze can propel droplets 5 feetThis 2009 photograph, from the CDC (Brian Judd/James Gathany) captured a sneeze in progress, revealing the plume of salivary droplets as they are expelled in a large cone-shaped array from this man’s open mouth. This is a perfect illustration of how easily the virus can travel from person to person. It also underlines the reason one needs to cover his/her mouth when coughing, or sneezing, in order to protect others from germ exposure.

Experts recommend avoiding people who are sick, or at the very least, staying 6 feet away from someone who might be infected to avoid exposure from the droplet spray of coughing, sneezing, or even talking.

Also, the influenza virus can live on surfaces, such as door knobs, TV controls, grocery cart handles, etc., for as long as 48 hours, depending on the virus and the surface. Touching that surface and then touching your food or face can transmit the virus into your respiratory system. Keeping your hands away from your mouth and nose, washing hands frequently, and before meals especially, will greatly help to prevent the spread of flu germs into your respiratory tract.

Another important factor that will influence whether or not you catch the flu involves your body's own resistance to the virus. Getting enough sleep, eating a well-balanced diet, exercising regularly, and most importantly, getting the flu shot every year will greatly help your body fight off any incoming hostile germs.

What is the incubation period for the flu?

The typical incubation period for influenza is 1 to 4 days, with an average of 2 days. Adults can be infectious from the day before symptoms begin through approximately 5 days after illness onset. Children can be infectious for >10 days after the onset of symptoms. Severely immunocompromised persons can shed the virus for weeks or months.

Symptoms: Influenza symptoms usually come on suddenly and hit hard:In bed with the flu

          • fever (usually high)
          • headache
          • muscle aches
          • extreme tiredness
          • dry cough
          • sore throat
          • head congestion with a runny or stuffy nose

Stomach symptoms, such as nausea, vomiting, and diarrhea, are not typical symptoms of influenza, but can occur. These symptoms are more common in children than adults.

How long does the flu last?

After 5 days, fever and other symptoms have usually disappeared, but a cough and weakness may continue. All symptoms related to the influenza virus are usually gone within a week or two. Complications from the flu, however, can worsen or prolong symptoms and require medical attention:

  • Secondary bacterial infections can set in as the flu is resolving. A common sign of this occurring is when someone who has been sick from the flu, starts to feel better, then starts feeling sick again. In this case, a secondary bacterial infection may be setting in causing either sinusitis, ear infections, bronchitis, or pneumonia.
  • Dehydration also can occur as a result of a high fever, excessive sweating and drinking inadequate fluids. (Vomiting or diarrhea, if present, can also be a cause.) Dehydration can occur in anyone, but it is most serious in babies, small children, and older adults.
  • Chronic medical conditions, such as congestive heart failure, asthma, or diabetes, can worsen as a complication of influenza.

Influenza can lead to life-threatening complications particularly in older people, young children, and people with chronic medical conditions. Every year in the United States more than 200,000 people are hospitalized from flu complications, and about 36,000 people die from the flu.

Read More:

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Written by N Thompson, MSN, ARNP, Last updated January 2009

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