Flu (influenza), seasonal

Seasonal flu (also known as influenza) is a highly infectious illness caused by the flu virus.

The virus infects your lungs and upper airways, causing a sudden high temperature and general aches and pains.

You may also lose your appetite, feel nauseous and have a dry cough. You may need to stay in bed until your symptoms get better.

Symptoms can last for up to one week.

How it is spread

The flu virus is spread in the small droplets of saliva coughed or sneezed into the air by an infected person. If you breathe in these droplets, you may become infected.

Flu can also spread if someone with the virus touches common surfaces such as a door handle with unwashed hands.

Typically several different strains of flu virus circulate at the same time. In 2010-11 one of the season's strains was H1N1, responsible for swine flu.

The infectious period

Symptoms develop one to four days (two days on average) after being infected.

People with flu are usually infectious (can spread the virus) a day before symptoms start, and remain infectious for five or six days. Children and people with weaker immune systems (such as cancer patients) may remain infectious for slightly longer.

Try to avoid all unnecessary contact with others during this infectious period.

How common is it?

Seasonal flu is a very common illness that occurs every year, usually during the winter months (October to April in Ireland).

The number of people who consult their GP with flu-like symptoms varies from year to year, but is usually between 50 and 200 for every 100,000 people. This is in addition to the many people with flu who do not see their GP.

Outlook

Your symptoms will usually peak after two to three days. You should begin to feel much better within five to eight days.

However, elderly people or those with certain medical conditions may develop pneumonia. This can lead to serious illness and can be life-threatening.

Pregnant women are more likely to have complications if they become ill with the flu.

The seasonal flu vaccination is recommended for everyone aged 65 and older, pregnant women, anyone over 6 months who has a long-term illness, and health care staff and carers.

Yearly flu vaccination is the best way to protect against influenza.

Immune system

The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.

Epidemic

An epidemic is a sudden outbreak of disease that spreads through a population in a short amount of time.

Seasonal flu can give you any of these symptoms:

  • sudden fever (a temperature of 38°C/100.4°F or above),
  • dry, chesty cough,
  • headache,
  • tiredness,
  • chills,
  • aching muscles,
  • limb or joint pain,
  • diarrhoea or stomach upset,
  • sore throat,
  • runny or blocked nose,
  • sneezing,
  • loss of appetite, and
  • difficulty sleeping.

Your symptoms will usually peak after two to three days and you should begin to feel much better within five to eight days. A cough and general tiredness may last for two to three weeks.

Glossary

Congested
Congestion is an excess of fluid in part of the body, often causing a blockage.
Nausea
Nausea is when you feel like you are going to be sick.
Drowsy
Drowsiness is when someone feels extremely tired and uncontrollably near to sleep.
Fever
A high temperature, also known as a fever, is when someone's body temperature goes above the normal 37°C (98.6°F).
Vomiting
Vomiting is when you bring up the contents of your stomach through your mouth.
Sneezing
Sneezing is an involuntary expulsion of air and bacteria from the nose and mouth.
Aches
An ache is a constant dull pain in a part of the body.
Diarrhoea
Diarrhoea is the passing of frequent watery stools when you go to the toilet.
Lungs
Lungs are a pair of organs in the chest that control breathing. They remove carbon dioxide from the blood and replace it with oxygen.

Flu or cold?

Many of the symptoms of flu are similar to those of the common cold, and many people incorrectly refer to a heavy cold as flu.

  • Symptoms of a cold appear gradually, are not severe and affect just your nose and throat.
  • Symptoms of flu are more severe, causing fever and aching muscles.You will not be able to do your usual activities. Flu symptoms can begin suddenly.

The flu virus is contained in the millions of tiny droplets that come out of the nose and mouth when someone coughs or sneezes.

These droplets typically spread about one metre (3ft). They hang suspended in the air for a while but then land on surfaces, where the virus can survive for up to 24 hours.

Anyone who touches these surfaces can spread the virus by touching anything else.

Everyday items at home and in public places may have traces of the virus. These include food, door handles, the remote control, handrails and computer keyboards.

People usually become infected by picking up the virus on their hands from contaminated objects and then placing their hands near their mouth or nose. It is also possible to breathe in the virus if it is suspended in airborne droplets.

Resistance and mutation

If you become infected with a flu virus your body will produce antibodies against it. Antibodies are proteins that recognise and fight off germs that have invaded your blood, such as viruses. 

Your antibodies will remember this flu virus and fight it if it invades your body again.

Over time the flu virus can mutate (change) into a different version, which means your body may not recognise it and you can catch flu again in the future.

The flu virus mutates because its structure is unstable. When the virus mutates to a new strain that people have little or no resistance to, it can cause a flu pandemic, which means it can spread globally. 

Glossary

Immunity
The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.
Epidemics
An epidemic is a sudden outbreak of disease that spreads through a population in a short amount of time.

Types of flu

There are three main types of flu:

  • Type A occurs every year and is more serious than type B. The virus is likely to mutate to a different version to which people have no resistance.
  • Type B generally causes a less severe illness and is responsible for smaller outbreaks. If you have been infected with this, your immunity to further flu B infections may last for many years. Flu B can affect all ages but mainly affects young children. Type B doesn't cause pandemics.
  • Type C usually causes a mild illness similar to the common cold.

Typically, every year one or two strains of type A flu may be circulating, as well as a type B strain.

Healthy people with seasonal flu do not need to contact their GP as they will get better on their own by taking over-the-counter flu remedies, resting, and drinking plenty of fluids.

When you should see your GP

See your GP if any of the following applies to you:

  • Your symptoms have got much worse, or you have developed other symptoms that are not typical of seasonal flu, such as a rash.
  • Your symptoms have lasted for longer than a week.
  • You have a medical condition that is making your flu worse.

Flu is usually diagnosed based on your symptoms, but your GP will also ask for your medical history. Your GP may also take a nose and throat swab if you are very ill or your condition has got worse.

If another disease is suspected (for example malaria if you have recently been travelling), then other tests or a referral may be necessary.

If you are otherwise fit and healthy, you can manage your symptoms of seasonal flu at home (see below). You will usually get better without treatment.

If you are in an at-risk group (see box) and have flu-like symptoms, or if your flu symptoms are getting worse, see your GP. These groups are more likely to suffer complications from flu. You may be prescribed antiviral medication (see below).

Antibiotics are not prescribed for flu as they have no effect on viruses. However, antibiotics may be required to treat complications of flu, especially serious chest infections.

Caring for yourself at home

Make sure you have plenty of rest and are taking paracetamol-based cold remedies to lower your temperature and relieve symptoms.

Some over-the-counter treatments can be given to children who have flu, according to the instructions supplied with each medicine. Under-16s must not take aspirin or ready-made flu remedies containing it.

Always read the medicine's label or check with the pharmacist that it is suitable for children to take.

Drink plenty of fluids while you are recovering. You may need to stay in bed for two or three days after your symptoms peak.

Glossary

Chronic
Chronic usually means a condition that continues for a long time or keeps coming back.
Antiviral
Antiviral medicine is used to treat a viral infection. For example, interferon.
Immune system
The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.
Acute
Acute means occuring suddenly or over a short period of time.
Antibiotics
Antibiotics are medicines that can be used to treat infections caused by micro-organisms, usually bacteria or fungi. For example amoxicillin, streptomycin and erythromycin.
Epidemic
An epidemic is a sudden outbreak of disease that spreads through a population in a short amount of time.

Bacterial infection

The most common complication of seasonal flu is a bacterial chest infection. Occasionally, this can become serious and develop into pneumonia.

A course of antibiotics usually cures the bacterial infection, but it can very occasionally become life-threatening, particularly in the frail and elderly.

Other serious complications are uncommon.

Rare complications

Rare complications include:

 

  • tonsillitis
  • otitis media (a build-up of fluid in the ear),
  • septic shock (infection of the blood that causes a severe drop in blood pressure),
  • meningitis 
  • (infection in the brain and spinal cord), and
  • encephalitis (inflammation of the brain).

Glossary

Kidney
Kidneys are a pair of bean-shaped organs located at the back of the abdomen, which remove waste and extra fluid from the blood and pass them out of the body as urine.
Immune system
The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.
Acute
Acute means occurring suddenly or over a short period of time.
Antibiotics
Antibiotics are medicines that can be used to treat infections caused by micro-organisms, usually bacteria or fungi. For example amoxicillin, streptomycin and erythromycin.
Heart
The heart is a muscular organ that pumps blood around the body.
Lung
Lungs are a pair of organs in the chest that control breathing. They remove carbon dioxide from the blood and replace it with oxygen.
Fever
A high temperature, also known as a fever, is when someone's body temperature goes above the normal 37°C (98.6°F).

At-risk groups

The following groups of people have a greater chance of developing complications and serious illness from flu:

  • people aged 65 and older,
  • people with certain medical conditions (long-term heart, lung, kidney, liver or neurological disease, diabetes or those with a weakened immune system due to treatment or disease), and
  • people living in nursing or residential homes where the spread of the virus is more likely.

Seasonal flu jab

The flu vaccine is given free of charge* by GPs, pharmacies or occupational health departments to the following at-risk people, to protect them from seasonal flu:

Vaccination is strongly recommended for:

  • Persons aged 65 and over
  • Those aged 6 months and older with a long-term health condition such as
    Chronic heart disease
    Chronic liver disease
    Chronic renal failure
    Chronic respiratory disease, including cystic fibrosis, moderate or severe asthma or bronchopulmonary dysplasia
    Chronic neurological disease including multiple sclerosis, hereditary and degenerative disorders of the central nervous system
    Diabetes mellitus
    Down syndrome
    Haemoglobinopathies
    Morbid obesity i.e. body mass index over 40
    Immunosuppression due to disease or treatment, including asplenia or splenic dysfunction
  • Children aged 6 months and older
    with any condition (e.g. cognitive dysfunction, spinal cord injury, seizure disorder, or other neuromuscular disorder) that can compromise respiratory function especially those attending special schools/day centres with moderate to severe neurodevelopmental disorders such as cerebral palsy and intellectual disability
    on long-term aspirin therapy (because of the risk of Reyes syndrome)
  • Pregnant women (vaccine can be given at any stage of pregnancy)
  • Healthcare workers
  • Residents of nursing homes and other long stay institutions
  • Carers
  • People with regular contact with pigs, poultry or water fowl

* The vaccine and consultation are free to those within the recommended groups who have a 'Medical Card' or 'Doctor Only Card'. Family doctors and Pharmacists charge a consultation fee for seasonal flu vaccine to those who do not have a 'Medical Card' or 'Doctor Only Card'.

Antiviral medication

The UK National Institute for Health and Clinical Excellence (NICE) recommends the antiviral medicines oseltamivir and zanamivir to prevent flu if all of the following apply:

  • The amount of flu virus circulating is sufficient to mean that if someone has a flu-like illness, it is likely to have been caused by this flu virus.
  • The person has a certain medical condition (see box) or is over 65.
  • The person has been in contact with someone with a flu-like illness and can start treatment within 36 hours (zanamivir) or within 48 hours (oseltamivir).
  • The person has not been effectively protected by vaccination (see below).

People who are not effectively protected by vaccination include:

  • Those who have not been vaccinated since the previous winter.
  • Those who cannot be vaccinated, or who have been vaccinated but it has not taken effect yet.
  • Those who have been vaccinated for a different form of flu virus.

If there is an outbreak of seasonal flu in a residential or nursing home, oseltamivir and zanamivir may be offered to people if they have been in contact with someone with confirmed flu. This is because these homes are closed places in which flu can spread quickly.

Glossary

Chronic
Chronic usually means a condition that continues for a long time or keeps coming back.
Immune system
The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.
Liver
The liver is the largest organ in the body. Its main jobs are to secrete bile (to help digestion), detoxify the blood and change food into energy.
Kidney
Kidneys are a pair of bean-shaped organs located at the back of the abdomen, which remove waste and extra fluid from the blood and pass them out of the body as urine.
Heart
The heart is a muscular organ that pumps blood around the body.
Vaccination
Vaccination or immunisation is usually given by an injection that makes the body's immune system produce antibodies that will fight off a virus.

Good hygiene

Preventing the spread of germs is the most effective way to slow the spread of flu. Always:

  • ensure you wash your hands regularly with soap and water,
  • clean surfaces regularly to get rid of germs,
  • use tissues to cover your mouth and nose when you cough or sneeze, and
  • put used tissues in a bin as soon as possible.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

Browse Health A-Z