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All you need to know about flu

All you need to know about flu

By Justin Choi, MD

Influenza, or flu, is a respiratory illness caused by a virus. Flu is highly contagious and is normally spread by the coughs and sneezes of an infected person.

A person can also catch flu by touching an infected person, for instance, by shaking hands.

Adults are contagious 1–2 days before getting symptoms and up to 7 days after becoming ill. This means that you can spread the influenza virus before you even know you are infected.

In this article, we explain the symptoms of flu, how it is treated, how it differs from a cold, and the best ways to prevent flu occurring.

Fast facts on flu

Here are some key points about flu.

  • Antibiotics cannot treat flu, but some antivirals can.
  • Approximately 5–20 percent of people will have flu at some time.
  • Experts agree that the best way to prevent flu is to get vaccinated each year.
  • The flu vaccine is not suitable for certain groups of people, such as those who have a severe allergy to chicken eggs.


Confusing flu with a bad cold is common.

Flu and cold symptoms may both include:

  • a runny or blocked nose
  • a sore throat
  • a cough

To help you tell them apart, below are some symptoms of flu that are different from a heavy cold:

  • a high temperature
  • cold sweats and shivers
  • headache
  • aching joints and limbs
  • fatigue, feeling exhausted

There may also be gastrointestinal symptoms, such as nausea, vomiting, and diarrhea. These are more common in children than in adults.

Normally, symptoms linger for about 1 week. However, the feeling of tiredness can continue for several weeks.

It is worth noting that not every person with flu will have all of the symptoms. For instance, it is possible to have flu without a fever.

Early symptoms of flu
Often, fatigue is one of the earliest signs of flu and cold. With flu, the fatigue tends to be more extreme.

Other early symptoms can include:

  • a cough
  • a sore throat
  • a fever
  • body aches
  • chills
  • gastrointestinal changes

A virus causes flu, so antibiotics cannot help, unless the flu has led to another illness caused by bacteria.

A doctor may prescribe antivirals, such as oseltamivir (Tamiflu) and zanamivir (Relenza), in some circumstances.

In addition, in 2018, the United States Food and Drug Administration (FDA) approved a new drug—baloxavir marboxil (Xofluza)—for acute uncomplicated influenza, or flu. People will take the drug by mouth, in a single dose.

People can receive this treatment if they:

  • are aged 12 years or over
  • have had symptoms for fewer than 48 hours

Possible side effects include diarrhea and bronchitis.

Pain relief medication
Pain relievers can alleviate some of the symptoms, such as a headache and body pains.

Various painkillers are available to purchase online. It is important to compare different products, and only take them under the advice of a medical professional.

Some painkillers, such as aspirin, are not suitable for children under 12 years of age.

Home remedies
Individuals with flu should:

  • stay at home
  • avoid contact with other people where possible
  • keep warm and rest
  • consume plenty of liquids
  • avoid alcohol
  • stop smoking
  • eat if possible

It is a good idea for people that live alone to tell a relative, friend, or neighbor that they have flu and make sure someone can check in on them.

When to see a doctor
A doctor only needs to know that a person has the flu if:

  • they are frail or older in age
  • their temperature remains high after 4–5 days
  • symptoms worsen
  • they feel seriously ill
  • they become short of breath, develop chest pain, or both

If worried, it may be better to call the doctor rather than making an appointment.

Flu is not usually serious, but it is unpleasant. For some people, however, there can be severe complications.

This is more likely in:

  • very young children
  • older people
  • individuals with other longstanding illness that can undermine their immune system

The risk of experiencing severe flu complications is higher for certain people:

  • adults over 65
  • babies or young children
  • pregnant women
  • individuals with heart or cardiovascular disease
  • those with chest problems, such as asthma or bronchitis
  • individuals with kidney disease
  • people with diabetes
  • people taking steroids
  • individuals undergoing treatment for cancer
  • those with longstanding diseases that reduce immune system function

The complications of influenza may include:

  • bacterial pneumonia
  • dehydration
  • worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes

Children may develop sinus problems and ear infections.

In the United States, over 200,000 people spend time in the hospital with flu complications each year, and about 36,000 people are estimated to die as a result of flu.

It is estimated that, globally, 250,000-500,000 people die each year as a result of flu.

In industrialized countries, the majority of deaths occur among people over the age of 65.

A flu epidemic—where a large number of people in one country are infected—can last several weeks.

Health experts and government agencies throughout the world say that the single best way to protect oneself from catching flu is to have a flu vaccination every year.

There are two types of vaccinations:

  • the flu shot
  • the nasal-spray flu vaccine

A healthcare professional will administer the flu shot with a needle, usually in the arm. It has approval for anyone older than 6 months, including healthy people and those with chronic medical conditions.

The nasal-spray flu vaccine is made with live, weakened flu viruses that do not cause illness.

Seasonal flu shot
A flu vaccine will contain three influenza viruses:

  • influenza (H3N2) virus
  • influenza (H1N2) virus
  • one B virus

As viruses adapt and change, so do those contained within the vaccines. Their content is based on international surveillance and scientists’ calculations about which virus types and strains will circulate in a given year.

Protection begins about 2 weeks after receiving the vaccination.

Seasonal flu vaccinations should start in September or as soon as the vaccine is on hand, and continue throughout the flu season, into January, and beyond. This is because the timing and duration of influenza seasons are never the same.

Flu outbreaks usually peak at around January, but they can happen as early as October.

Seasonal flu shots are not suitable for some people
Certain individuals should check with their doctor before deciding to have the flu vaccine, including:

  • Individuals with a severe allergy to chicken eggs.
  • Individuals who have had a severe reaction to a flu vaccination in the past.
  • Individuals who developed Guillain-Barré Syndrome within 6 weeks of receiving a flu vaccine.
  • Children under 6 months old.
  • Individuals experiencing a fever with a moderate-to-severe illness should wait until they recover before being vaccinated.

There are three types of flu virus:

  • influenza A
  • influenza B
  • influenza C

Types A and B viruses cause seasonal epidemics in the U.S. and Europe virtually every winter.

The type C influenza virus causes mild respiratory illness and is not responsible for outbreaks.

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