Flu symptoms

What are the symptoms of flu, how long do they last and when should I see a doctor?

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Immune System Expert
Ask Dr. Jen Tan

What are the symptoms of the flu?

Flu is caused by the influenza viruses. Although there are a few types of flu viruses, the same symptoms generally appear. These include:

These are very much the same symptoms as may be experienced with colds. However, in general, flu gives rise to more severe symptoms and a severe cold may sometimes be treated as flu.

Occasionally, severe flu symptoms may be experienced with the development of serious medical conditions or complications.

How long will symptoms last?

The length of flu infection will depend on the individual. Generally speaking, symptoms persist for about a week, although this can be much longer with a severe infection or in someone with a weakened immune system.

The first indication that you have the flu is often a sudden high temperature and fever.  You may feel that you have achy muscles. Your temperature will probably go down within 48 hours.

After this, your symptoms may become more like a cold. You might suffer from a sore throat, and a dry cough. This is because your immune system is now releasing chemicals to attack the flu virus, but these chemicals irritate your respiratory system causing these symptoms. You may also start to have a runny nose. This reaction removes any cells from your body which your immune system has already killed.

You may begin to feel congested with a blocked nose after 3 or 4 days. This happens when the mucous discharge becomes thicker. This can cause a chesty cough as your body tries to get rid of the excess mucus. You may also experience a headache.

Eventually, by the end of the week, your symptoms should begin to subside and you will begin to feel like your normal self again. However, it can take a couple of weeks before you feel that you have fully recovered.

How do I know whether it’s a cold or the flu?

It can sometimes be difficult to tell whether you have a bad cold or the flu.

To say that flu is caused by the influenza virus, and that a cold is caused by one of about 200 different viruses, is really no help at all. However, a look at the symptoms and how they arrive might make it easier to establish whether you are suffering from the cold or flu.

Cold symptoms generally appear gradually. You might have a sore throat for a couple of days before any other symptoms develop. With the flu, however, all the symptoms will hit you like a brick wall from day one. You are more likely to suffer a fever with the flu, and diarrhoea and vomiting are not uncommon.

Generally, it will not take long after your cold symptoms have gone to be up and running as usual. However, with the flu, you may still feel drained and fatigued for a week or more.

Complications of the flu

If you are normally healthy, then the flu may make you feel worn out while it runs its course, but it should not present any medical complications.

However, sometimes a complication of flu will develop, particularly if you have a weakened immune system. These include:

  • Bronchitis – this is an inflammation of the tubes which carry air to and from your lungs. It makes it difficult to breathe, and can often result in a chesty, mucous cough. This can develop into pneumonia
  • Pneumonia – sometimes a viral or bacterial chest infection can lead to pneumonia. This is the most common complication of the flu. It occurs when lung tissue becomes inflamed, making it difficult to breathe and is likely to need treatment in hospital
  • Ear infections – the most common type of ear infection is when fluid and pus build up in the middle ear. This is most common in children after a bout of cold or flu. It may clear up by itself after a few days, or need antibiotics to clear the infection.

When to see the doctor

Weak immune function makes it more difficult for the body to withstand infection, so an early sign of a weak immune system is a tendency for the body to pick up infections such as colds and flu more easily.

This happens because a poorly functioning immune system can’t spot the nasty invaders efficiently. In addition, the immune system may also take longer to overcome bugs.

It is important to remember that people with weak immune systems such as children, the elderly, and those with underlying medical problems may suffer more severe symptoms with the flu, and are more likely to need medical attention.

You should seek medical attention if you experience any of the following symptoms:

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