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Influenza is a common and potentially serious viral infection, commonly called the flu, that causes upper respiratory symptoms. Influenza viruses rapidly adapt and mutate, which gives them the perpetual ability to cause illness.
There are three types of human influenza virus-influenza A, influenza B, and influenza C. Influenza A viruses are primarily responsible for annual outbreaks of the flu, though influenza B viruses also cause illness.
Some strains of influenza A infect humans and some strains infect animals such as pigs (swine influenza virus) and poultry (avian influenza virus). Strains of influenza A that infect animals can sometimes mutate in ways that permit them to cross over to infect people, as happened in 1918 with the world’s most severe influenza pandemic, known as the Spanish flu (because the first cases occurred in Spain), which researchers today believe mutated from a variety of swine influenza virus.
Symptoms of Influenza and Diagnostic Path
Influenza is an illness of the upper respiratory tract, the symptoms of which tend to emerge rapidly and full-force.
Symptoms of influenza include:
- High fever (102ºF or higher)
- Severe headache
- Muscle aches and joint pain
- Nonproductive cough
- Sore throat
- Nasal and sinus congestion
Some people, especially young children, may experience nausea and vomiting. However, gastrointestinal symptoms are not characteristic of influenza, and their prominent presence suggests a different infection. There are several tests available for influenza, including rapid tests that the doctor can use in the office as well as blood tests.
Influenza Treatment Options and Outlook
Most often, the flu simply runs its course and treatment targets relieving symptoms such as fever and aches. Rest, fluids, and nutritious foods are important for helping the body to fight the virus. Secondary bacterial infections and other complications can occur and require appropriate treatment.
Antibiotic medications are not effective for treating viruses, though the doctor may prescribe an antibiotic to treat a secondary bacterial infection that develops, such as pneumonia or strep throat. The most common complication of influenza is pneumococcal pneumonia, a bacterial infection for which there is a one-time vaccine (though people who have lung disorders or respiratory compromise may need a second vaccine 10 years after the first vaccine).
The flu vaccine
Immunization remains the frontline of treatment for influenza. Each year researchers determine the two strains of influenza A and one strain of influenza B most likely to cause infection (based on complex algorithms of historic and projected viral cycles).
Manufacturers then cultivate the three strains to create the year’s vaccine. This is a somewhat speculative approach, however, and the actual strains of influenza that surface may be entirely different. When the strains are similar to those in the vaccine the vaccine is highly effective in preventing or moderating influenza infection.
When the strains are not at all close, the vaccine offers no protection from influenza infection. Because the influenza virus so rapidly mutates, each vaccine is effective only for a single flu season.
Antiviral medications may shorten the course of illness and lessen the severity of symptoms when taken within 48 hours of the first symptoms and may prevent infection with influenza after exposure to someone who has the viral infection.
Antiviral medications work by interfering with the mechanisms viruses use to alter the functions of their host cells, typically by blocking the action of key enzymes or proteins that the virus uses to instruct RNA to take over the host cell.
Some antiviral medications are effective against influenza A (amantadine and rimantadine) and others against influenza B (zanamivir and oseltamivir). Further testament to the influenza virus’s ability to mutate is the emergence of influenza virus strains that are resistant to amantadine and rimantadine.
Risk Factors and Preventive Measures
People at highest risk for influenza are the very young, the very old, and those who have compromised immune function, such as people who have hiv/aids or cancer or who take immunosuppressive therapy after organ transplantation. People who have diabetes and other chronic health conditions also have increased risk for infection. Preventive measures to reduce the risk for influenza infection include
- Frequent hand washing with warm water and soap
- Disinfecting common use surfaces such as doorknobs
- Minimizing exposure to enclosed crowds of people during peak flu season (November through March in the United States)
Annual flu vaccines are currently the most effective method for preventing influenza.
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