Flu - World Health Organisation

Source: World Health Organization + www.flu.gov

What is flu?

Flu is a viral infection that affects mainly the nose, throat, bronchi and, occasionally, lungs. Infection usually lasts for about a week, and is characterized by sudden onset of high fever, aching muscles, headache and severe malaise, non-productive cough, sore throat and rhinitis.

The virus is transmitted easily from person to person via droplets and small particles produced when infected people cough or sneeze. Flu tends to spread rapidly in seasonal epidemics.

Most infected people recover within one to two weeks without requiring medical treatment. However, in the very young, the elderly, and those with other serious medical conditions, infection can lead to severe complications of the underlying condition, pneumonia and death.

Pandemic (H1N1) 2009 is a new Flu virus that has never circulated among humans before. After outbreaks in North America early in 2009, the virus spread rapidly around the world. Pandemic Flu is transmitted like seasonal Flu but people have virtually no immunity to it. Mitigating its effects is a public health priority.

Some groups are more likely to have complications from the seasonal flu. These include:

  • those age 65 and older
  • children younger than 2 years old
  • people of any age who have chronic medical conditions (e.g. diabetes, asthma, congestive heart failure, lung disease)

Complications from the flu can include:

  • bacterial pneumonia
  • ear or sinus infections
  • dehydration
  • worsening of chronic medical conditions

Different types of flu

1. Flu (Seasonal)

Key facts

  • Flu is an acute viral infection that spreads easily from person to person.
  • Flu circulates worldwide and can affect anybody in any age group.
  • Flu causes annual epidemics that peak during winter in temperate regions.
  • Flu is a serious public health problem that causes severe illnesses and deaths for higher risk populations.
  • An epidemic can take an economic toll through lost workforce productivity, and strain health services.
  • Vaccination is the most effective way to prevent infection.


Seasonal Flu is an acute viral infection caused by a Flu virus.

There are three types of seasonal Flu – A, B and C. Type A Flu viruses are further typed into subtypes according to different kinds and combinations of virus surface proteins. Among many subtypes of Flu A viruses, currently Flu A (H1N1) and A(H3N2) subtypes are circulating among humans. Flu viruses circulate in every part of the world. Type C Flu cases occur much less frequently than A and B. That is why only Flu A and B viruses are included in seasonal Flu vaccines.

Signs and symptoms

Seasonal Flu is characterized by a sudden onset of high fever, cough (usually dry), headache, muscle and joint pain, severe malaise (feeling unwell), sore throat and runny nose. Most people recover from fever and other symptoms within a week without requiring medical attention. But Flu can cause severe illness or death in people at high risk (see below). The time from infection to illness, known as the incubation period, is about two days.

Flu Symptoms and Cold vs. Flu

Common Symptoms

Possible Symptoms

  • fever (usually high)
  • tiredness (can be extreme)
  • headache
  • dry cough
  • sore throat
  • runny or stuffy nose
  • muscle aches

These symptoms may occur, but are more likely in children than adults:

  • nausea
  • vomiting
  • diarrhea

Note that these additional symptoms may also be a sign of the H1N1 flu.

Who is at risk?

Yearly Flu epidemics can seriously affect all age groups, but the highest risk of complications occur among children younger than age two, adults age 65 or older, and people of any age with certain medical conditions, such as chronic heart, lung, kidney, liver, blood or metabolic diseases (such as diabetes), or weakened immune systems.


Seasonal Flu spreads easily and can sweep through schools, nursing homes or businesses and towns. When an infected person coughs, infected droplets get into the air and another person can breath them in and be exposed. The virus can also be spread by hands infected with the virus. To prevent transmission, people should cover their mouth and nose with a tissue when coughing, and wash their hands regularly.


Antiviral drugs for Flu are available in some countries and effectively prevent and treat the illness. There are two classes of such medicines, 1) adamantanes (amantadine and remantadine), and 2) inhibitors of Flu neuraminidase (oseltamivir and zanamivir). Some Flu viruses develop resistance to the antiviral medicines, limiting the effectiveness of treatment. WHO monitors antiviral susceptibility in the circulating Flu viruses.


Seasonal epidemics

Flu epidemics occur yearly during autumn and winter in temperate regions. Illnesses result in hospitalizations and deaths mainly among high-risk groups (the very young, elderly or chronically ill). Worldwide, these annual epidemics result in about three to five million cases of severe illness, and about 250 000 to 500 000 deaths. Most deaths associated with Flu in industrialized countries occur among people age 65 or older. In some tropical countries, Flu viruses circulate throughout the year with one or two peaks during rainy seasons.

Disease effects

Flu can cause serious public health and economic problems. In developed countries, epidemics can result in high levels of worker absenteeism and productivity losses. In communities, clinics and hospitals can be overwhelmed when large numbers of sick people appear for treatment during peak illness periods. While most people recover from a bout of Flu, there are large numbers of people who need hospital treatment and many who die from the disease every year. Little is known about the effects of Flu epidemics in developing countries.


The most effective way to prevent the disease or severe outcomes from the illness is vaccination. Safe and effective vaccines have been available and used for more than 60 years. Among healthy adults, Flu vaccine can prevent 70% to 90% of Flu-specific illness. Among the elderly, the vaccine reduces severe illnesses and complications by up to 60%, and deaths by 80%.

Vaccination is especially important for people at higher risk of serious Flu complications, and for people who live with or care for high risk individuals.

WHO recommends annual vaccination for (in order of priority):

  • nursing-home residents (the elderly or disabled)
  • elderly individuals
  • people with chronic medical conditions
  • other groups such as pregnant women, health care workers, those with essential functions in society, as well as children from ages six months to two years.

Flu vaccination is most effective when circulating viruses are well-matched with vaccine viruses. Flu viruses are constantly changing, and the WHO Global Flu Surveillance Network (GISN), a partnership of National Flu Centres around the world, monitors the Flu viruses circulating in humans. WHO annually recommends a vaccine composition that targets the three most representative strains in circulation.

2. H1N1

What is H1N1 Flu?

  • H1N1 flu is a new influenza virus causing illness in people. It has two genes from flu viruses that normally circulate in pigs in Europe and Asia, plus avian genes and human genes. Scientists call this a "quadruple reassortant" virus. Read more about where this virus came from in this Q&A on the origin of the H1N1 Flu.
  • H1N1 flu is contagious. This new virus was first detected in people in the United States in April 2009. The virus is spreading from person-to-person, in the same way that regular seasonal influenza viruses spread.
  • H1N1 flu is NOT caused by eating pork or pork products. H1N1 flu is not a foodborne disease, it is a respiratory disease. The USDA continues to remind consumers that all meat and poultry products are safe to eat when properly prepared and cooked.
  • Illness with the new H1N1 flu virus has ranged from mild to severe. While the vast majority of people who have contracted H1N1 flu have recovered without needing medical treatment, hospitalizations and deaths have occurred.
  • About 70 percent of people who have been hospitalized with H1N1 flu have had one or more medical conditions that placed them in the "high risk" category for serious seasonal flu-related complications. These include pregnancy, diabetes, heart disease, asthma and kidney disease.
  • Seniors (adults 65 years and older) are prioritized for antiviral treatment to limit risk of complication if they get flu. While your age means you have a lower risk of getting the flu, certain risk conditions (COPD, diabetes, etc.) mean if you get sick, you may have higher risk of complications from any influenza.

Symptoms of H1N1 Flu

  • The symptoms of H1N1 flu are similar to seasonal flu, but may include additional symptoms such as vomiting and diarrhea.
  • Symptoms of Seasonal and H1N1 Flu:

    Seasonal Flu

    H1N1 Flu

    All types of flu can cause:

    Coughing and/or sore throat
    Runny or stuffy nose
    Headaches and/or body aches

    Similar to seasonal flu, but symptoms may be more severe.
    There may be additional symptoms. A significant number of H1N1 flu cases:


  • Emergency Warning Signs of H1N1 - If you become ill and experience any of the following warning signs, seek emergency medical care.

3. H5N1 (Bird Flu)

What is H5N1 (Bird) flu?

  • H5N1 (Bird) flu virus is an influenza A virus subtype that is highly contagious among birds, and can be deadly to them. The H5N1 (Bird) flu virus does not usually infect people, but rare infections with these viruses have occurred in humans. Nearly all human cases have resulted from people having direct or close contact with H5N1-infected poultry or H5N1-contaminated surfaces.
  • Symptoms of the H5N1 (Bird) flu virus in humans have ranged from typical human influenza-like symptoms (e.g., fever, cough, sore throat, and muscle aches) to eye infections, pneumonia, severe respiratory diseases (such as acute respiratory distress), and other severe and life-threatening complications. The symptoms of H5N1 (Bird) flu may depend on which virus caused the infection.

H5N1 (Bird) Flu Prevention and Treatment

  • Most cases of H5N1 (Bird) flu infection in humans have resulted from direct or close contact with infected poultry or surfaces contaminated with secretions and excretions from infected birds.
  • The U.S. government carefully controls domestic and imported food products, and in 2004 issued a ban on importation of poultry from countries affected by avian influenza viruses, including the H5N1 (Bird) flu strain. This ban still is in place.
  • You cannot get H5N1 (Bird) flu from properly handled and cooked poultry and eggs. Even if poultry and eggs were to be contaminated with the virus, proper cooking would kill it.

    Follow the same advice you always would for properly cooking eggs and poultry:

    Wash your hands with soap and warm water for at least 20 seconds before and after handling raw poultry and eggs.

    Clean cutting boards and other utensils with soap and hot water to keep raw poultry from contaminating other foods.

    Use a food thermometer to make sure you cook poultry to a temperature of at least 165 degrees Fahrenheit. Consumers may wish to cook poultry to a higher temperature for personal preference.

    Cook eggs until whites and yolks are firm.

  • If you plan to visit any of the countries that have had confirmed human infection with H5N1 (Bird) flu, visit Avian Flu & Travelers from CDC for advice on travel preparation.
  • Some of the prescription medicines approved in the United States for human influenza viruses can be effective in treating H5N1 (Bird) flu in humans. H5N1 (Bird) flu is resistant to amantadine and rimantadine, two antiviral medications commonly used for influenza. Two other antiviral medications, Oseltamivir (TAMIFLU®) and Zanamivir (RELENZA®) would likely be effective in treating H5N1 (Bird) flu, but additional studies still need to be completed to demonstrate their effectiveness. It is important to note that H5N1 (Bird) flu infection in humans is very rare.
  • The seasonal influenza vaccine does not provide protection against avian influenza.

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