An infection of the upper respiratory tract (airways), commonly known as flu

  • Age, gender, genetics, and lifestyle are not significant factors

Influenza, also known as flu, is a contagious viral disease that tends to occur in epidemics during the winter but may occur any time of the year. The infection mainly affects the upper respiratory tract (airways).

Types of influenza

Many different viral infections can result in mild flu-like symptoms, but true influenza is caused by three types of influenza virus: A, B, and C. These viruses can affect animals and birds as well as humans.

Type A can affect birds, pigs, and humans, and is responsible for the influenza pandemics (widespread epidemics) that occur periodically. The 1918 Spanish flu pandemic, 1957 Asian flu pandemic, 1968 Hong Kong flu pandemic, and 2009 swine flu pandemic were all caused by type A influenza.

Type B affects humans almost exclusively but can also affect seals and ferrets. It typically causes only small, localized influenza outbreaks. Type C can affect humans, dogs, and pigs but is less common than the other types and usually causes only mild disease in children.

Types A and B are constantly changing their structure (mutating) and producing new strains to which few people have immunity. That is why it is recommended that certain groups get immunized every year against the particular strain that is circulating. Swine flu (H1N1) and bird flu (H5N1) are variants of type A influenza that originated in pigs and birds, respectively, and cause influenza in humans.

The influenza virus can be spread in several ways. Typically, it is transmitted in airborne droplets from the coughs and sneezes of infected people or by direct human-to-human contact. “Ordinary” seasonal influenza and swine flu can be spread in these ways. However, there have been no cases of human-to-human transmission of bird flu and it seems that this type can only be caught directly from infected birds.

What are the symptoms?

The symptoms of seasonal influenza develop 24–48 hours following infection. Many people believe that they have influenza when they have only a common cold, but symptoms develop abruptly and worsen rapidly in just a few hours. They include the following:

  • Fever (38°C/100.4°F or above), sweating, and shivering.

  • Cough.

  • Aches and pains.

  • Severe exhaustion.

  • Frequent sneezing, stuffy or runny nose, sore throat.

  • Headache.

  • Vomiting or diarrhoea.

The symptoms of swine flu are almost indistinguishable from those of seasonal influenza. They usually develop between 2 and 5 days after infection but may not appear for as long as 7 days. Typically, people with swine flu have a sudden onset of fever (a temperature of 38°C/100.4°F or higher) and at least two of the following symptoms:

  • Sudden onset of a cough.

  • Sore throat.

  • Headache.

  • Tiredness.

  • Runny nose.

  • Aches and pains.

  • Vomiting or diarrhoea.

For all types of influenza the symptoms usually last for a few days and then clear up, although tiredness and depression may be experienced after the other symptoms have disappeared.

Are there complications?

The most serious complication of influenza is infection of the lungs (see Pneumonia), which can be life-threatening to people in certain high-risk groups. These include babies and children under 5 years old; pregnant women; adults over 65; those with reduced immunity, such as people with HIV/AIDS and those undergoing chemotherapy or treatment with corticosteroids; people with chronic lung disease or asthma that has required medical treatment within the last 3 years; those with heart disease, diabetes mellitus or another metabolic disorder, chronic liver disease, cystic fibrosis, sickle-cell disease, kidney disease, or muscular dystrophy ; and people who have recently had a stroke or who have a nervous system disorder such as cerebral palsy, Parkinson’s disease, or multiple sclerosis.

What might be done?

The symptoms of all types of influenza can be relieved by resting in bed, drinking plenty of fluids, and following the advice for bringing down a fever. Paracetamol helps lower temperature and ease aches and pains; ibuprofen has the same effects but should not be taken by pregnant women. However, you should consult your doctor immediately if you are in a high-risk group or have another serious underlying illness, if you have breathing difficulty, if your condition suddenly gets worse, or if your symptoms are still worsening after 7 days (or 5 days for a child). Your doctor may prescribe the antiviral drugs oseltamivir (Tamiflu) or zanamivir (Relenza). These drugs may reduce the severity and duration of the illness. They are best taken within 48 hours of the onset of symptoms, but the earlier the better. Your doctor may also arrange for tests to check for another infection, such as pneumonia. If another infection is found, your doctor may prescribe antibiotics, and in certain cases admission to hospital may be advised.

If you think you may have swine flu, you should seek medical advice. If your symptoms do indicate swine flu, you may be offered oseltamivir. For most people in high-risk groups, it will be recommended that they take this drug; however, pregnant women will be offered zanamivir instead. In some cases, people in close contact with the disease but who have no symptoms may also be offered antiviral medication.

To reduce the spread of all types of influenza (and other infectious diseases) general hygiene measures are important: you should cover your nose and mouth with a tissue when coughing or sneezing and then dispose of the tissue promptly and carefully; wash your hands frequently with soap and water; and clean hard surfaces (such as door handles and work surfaces) frequently. You should also make sure any children in the household follow these measures. People with symptoms of influenza should stay at home until they feel better as this will also help prevent the spread of the disease.

What is the prognosis?

For normally healthy people who do not develop complications, most symptoms of influenza usually disappear after 6–7 days, although a cough may persist for over 2 weeks and tiredness may last longer. However, for a minority of people – principally those in high-risk groups – influenza may cause severe illness and serious complications, which may be fatal in some cases.

Can it be prevented?

Immunization can give effective protection against seasonal influenza. It is recommended for elderly people, those in high-risk groups (excluding babies under 6 months), and those likely to be exposed to influenza, such as health care workers or carers of the elderly. Immunization prevents infection in about two thirds of people who are vaccinated annually. However, the vaccine can never be completely effective because the viruses frequently mutate, and different strains are responsible for outbreaks each year. A vaccine against swine flu is also available. Antiviral medications reduce the severity and duration of influenza but cannot prevent it.

Self-help: Bringing Down a Fever

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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