Flu: Everything you need to know

Flu: Everything you need to know

Influenza, or flu, is a respiratory disease caused by a viral infection. Flu is highly contagious, and it spreads by droplets in the air. A individual may pass it on during a conversation or through physical contact, including shaking hands.

Each winter in the United States, and elsewhere, influenza A and influenza B trigger seasonal epidemics. Type C tends to cause mild respiratory disease.

Some influenza A strains, such as the “bird flu” virus H5N1, often infect humans and cause serious illness. Experts closely watch these strains as they try to predict how they’re going to change, and how they might affect people.

We explain the symptoms of flu in this article, the treatment options, how it varies from a cold and how to avoid flu.


A lady having runny nose
A person with the flu may experience a stuffy or runny nose.

individual with flu can experience the below Symptoms accordingThe Centers for Disease Control and Prevention (CDC) :

  • a high temperature that lasts 3–4 days
  • a stuffy or runny nose
  • cold sweats and shivers
  • aches that may be severe
  • headache
  • fatigue

Not everyone with flu will experience all those symptoms. For example, flu can occur without a fever.

Influenza symptoms usually develop unexpectedly. One person with flu will initially experience:

  • a high temperature
  • a stuffy or runny nose
  • a dry cough
  • cold sweats and shivers
  • aches that may be severe
  • a headache
  • fatigue, and a feeling of being unwell
  • a low appetite

Flu symptoms in adults

Adults with the following symptoms should seek medical help urgently:

  • breathing difficulties
  • pain or pressure in the chest or abdomen
  • dizziness, confusion, or loss of alertness
  • seizures
  • not urinating, which may indicate dehydration
  • severe pain, weakness, and unsteadiness
  • fever or cough that goes away and then comes back
  • a worsening of other existing health conditions

Flu symptoms in children

Children often have similar symptoms to adults, but gastrointestinal symptoms such as nausea, vomiting, and diarrhea can also be present.

If an infant has the following symptoms, emergency medical care is needed:

  • breathing difficulties
  • rapid breathing
  • bluish face or lips
  • chest pain or ribs pulling inward as they breathe
  • severe aches
  • dehydration, for example, not urinating for 8 hours and crying dry tears
  • lack of alertness or interaction with others
  • a fever above 104°F or any fever in a child under 12 weeks of age
  • a fever or cough that goes away but then comes back
  • a worsening of any other medical conditions

Should children have flu medication? Find out more here about Tamiflu and its effects on children.

Flu symptoms in babies

Flu can be dangerous for babies. If symptoms appear, a parent or caregiver should seek medical help.

A baby with flu may:

  • be very tired
  • have a cough and sore throat
  • have a stuffy or runny nose
  • have a fever of 100°F or more
  • have vomiting or diarrhea

The baby needs emergency medical attention if they:

  • do not want anyone to hold them
  • have a blue or gray skin color
  • are breathing fast or have difficulty breathing
  • have a fever with a rash
  • have symptoms that go away but come back again
  • show signs of dehydration, for example, not urinating
  • do not wake up or interact
  • have severe and persistent vomiting

Flu type A symptoms

If a person has the following symptoms, they may have influenza type A:

  • fever and chills
  • headache
  • muscle aches
  • fatigue
  • weakness
  • a stuffy or runny nose
  • a sore throat and cough

Flu type B symptoms

Influenza B symptoms are similar to those of influenza A.


Most people can treat influenza during home. A mixture of lifestyle treatments and over- the-counter drugs can help alleviate symptoms.

Medication for pain relief will help manage a headache and body pain. A healthcare professional can recommend the best options.

Many painkillers, for example aspirin, are not suitable for children under the age of 16. At this age the use of aspirin will lead to a condition known as Reye’s syndrome.

Numerous options are available over the counter. Comparing different products is necessary, and only taking them under a medical professional’s advice.

Flu medication

A virus causes flu and therefore antibiotics won’t cure the disease. A physician can prescribe antibiotics only when there is a bacterial infection alongside the flu. Antiviral drugs, however, can help when someone has the flu.

Antivirals aim to prevent replication of the virus in a person’s body. Take for example oseltamivir (Tamiflu) and zanamivir (Relenza).

In 2018, the Food and Drug Administration (FDA) approved a new drug for acute, uncomplicated flu, called baloxavir marboxil (Xofluza). People are allowed to take the drug in a single dose by mouth.

People may take this medication if they are 12 years of age or older and have symptoms for less than 48 hours. Possible side effects include bronchitis and diarrhea.

Flu home remedies

When a person has flu, it is essential that they:

  • stay at home
  • avoid contact with other people if possible
  • keep warm and rest
  • consume plenty of liquids and healthful foods
  • avoid alcohol
  • stop smoking, as this raises the risk of complications

Other things people can try at home include:

  • chicken broth
  • herbal teas
  • vitamin supplements

Nevertheless, insufficient evidence is available to prove that consuming such helps.

Flu diagnosis

If a person is seeking medical advice for flu symptoms, a doctor will probably inquire about their symptoms and will do a physical exam. A doctor can take a swab of the throat for testing too.

The rapid diagnostic test for influenza will yield results in 10–15 minutes but may not be reliable. Other, more comprehensive experiments will take longer to yield results.

Flu or a cold?

Flu vs a cold image
Image credit: Stephen Kelly, 2018.

People often confuse the flu with a bad cold, since there are similar symptoms.

A cold and the flu both involve:

  • a runny or blocked nose
  • a sore throat
  • a cough
  • chest discomfort
  • fatigue

However, there are some differences:

  • A cold does not involve a fever, while the flu usually does.
  • The symptoms of a cold tend to appear gradually, while flu symptoms can develop rapidly.
  • Cold symptoms are typically less severe than those of flu.
  • After having the flu, a person may continue to feel tired for several weeks.
  • Flu is more likely to lead to complications, and it can be life threatening.

Flu or food poisoning?

Many types of viruses exist, and some can affect the digestive system. Sometimes people call that “stomach flu.” This disease is different from influenza, which is a respiratory disease.

The norovirus, which comes into the body through contaminated food or drink, is the most common cause of “stomach flu.” Symptoms include nausea and vomiting, as well as diarrhea. Similar symptoms caused by food poisoning.

Flu or pneumonia?

Pneumonia is possibly bacterial or viral. The symptoms may resemble those of flu, but an individual may experience sharp, stabbing chest pain, especially when breathing deeply or coughing.

Bacterial pneumonia may begin slowly or all of a sudden. Could include symptoms like:

  • a very high temperature
  • sweating
  • rapid breathing and pulse rate
  • blue nailbeds due to a lack of oxygen

Symptoms of viral pneumonia are similar to those of the flu. They include:

  • fever
  • dry cough
  • headache
  • aches and weakness

Like flu, however, pneumonia symptoms usually grow slowly. Anyone having high fever and difficulty breathing should see a doctor right away.

When is flu season?

People can have flu at any time but during the flu season it is more popular. The timing and length of the flu season vary from year to year, but it usually occurs during winter and fall.

In October, flu activity always begins to rise, and can last as late as May. It’s most common, however, from December through February.


The flu shot can help prevent flu, but it is not 100% effective. People should follow lifestyle measures to reduce their risk.

Lifestyle tips for avoiding flu

Tips for avoiding infection include:

  • practicing good hygiene, including frequent hand washing
  • keeping the immune system strong by following a healthful diet
  • quitting or avoiding smoking, as smokers are more likely to develop complications
  • staying away from people who have the flu

People should stay away from others too when they have the flu to stop spreading it on their own.


The flu virus is spread by liquid droplets. When they cough, sneeze, speak or breathe, a person may pass the virus on to another person who is up to 6 feet away from them.

A healthy person will pass on the virus a day before they’re symptoms themselves. Or put it another way, you can pass on the flu until you know you have it. After symptoms appear, the infected individual can continue to transmit the virus for up to 5–7 days.

People with a weakened immune system, older people and young children may have the ability to spread the virus for longer.

In the first 3–4 days after symptoms appear, flu is the most infectious.


When droplets that contain the virus and come from another person’s breath penetrate their mouth, nose or lungs, a person can develop flu symptoms.

This transmission can happen if:

  • Someone without the virus is near a person with flu.
  • Someone who is virus free handles an object a person with the virus has touched and then touches their mouth, nose, or eyes.

Incubation period

A disease’s incubation period is the time it takes from when a person is infected with the virus to when symptoms begin.

This is around 2 days for flu but it can range from 1 to 4 days.

Even before symptoms appear a person can transmit the virus.

Flu when pregnant

Throughout pregnancy, flu may be more serious, because pregnancy affects how the immune system functions. If a woman is pregnant and has flu, they may need to be in the hospital for some time.

Pregnancy-related complications include a higher risk of:

  • preterm birth
  • low birth weight
  • stillbirth

For newborns, flu can be deadly. Mother’s risks include a higher likelihood of having complications, such as bronchitis, ear and blood infections.

How long does it last?

Symptoms of flu suddenly appear, usually around 2 days after infection. After about 1 week, most symptoms disappear but a cough may last for up to 2 weeks.

In some cases, a person may still be contagious for up to 1 week after the symptoms are gone.

Those can take longer to resolve if complications develop. Some of the more severe types of complications, such as kidney failure, can have a long term impact on a person’s health.

Some people experience post-viral fatigue for about a week after the principal symptoms have disappeared. They may have a lingering feeling of fatigue and feeling unwell.


Typically, flu may progress as follows:

  • The virus infects a person, usually through their nose or mouth.
  • After one day, they may be able to transmit the virus to others.
  • Symptoms appear 1–2 days after infection.
  • The chance of transmitting the virus is highest 3–4 days after symptoms appear.
  • After 4 days, the fever and muscle aches improve.
  • After 1 week, most symptoms disappear.
  • The risk of transmitting the virus disappears 5–7 days after symptoms appear.
  • The cough and tiredness may remain for a further week.


Normally the flu is not serious but it is uncomfortable. However complications may arise for some people. Some of those might be life-threatening.

Complications include:

  • bacterial pneumonia
  • dehydration
  • worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes
  • sinus problems and ear infections

The risk of experiencing severe symptoms and flu complications is higher in the following cases:

  • adults over 65 years of age
  • babies or young children
  • pregnant women
  • people with heart or cardiovascular disease
  • individuals with chest problems, such as asthma or bronchitis
  • people with kidney disease or diabetes
  • individuals who are taking steroids
  • people undergoing treatment for cancer
  • anyone with a weakened immune system

Sometimes a new type of flu will emerge, such as H5N1, or bird flu.

Flu shot

The single best way to prevent flu is to have a flu vaccination every year.

There are two types of vaccination:

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

The nasal-spray flu vaccine: The nasal-spray flu vaccine contains live, weakened flu viruses that do not cause illness.

Seasonal flu shot

A flu shot will contain the vaccine for several influenza viruses, according to the CDC.

Examples include:

  • influenza A (H1N1) virus
  • influenza (H3N2) virus
  • one or two influenza B viruses

Viruses, however, adapt and change over time and scientists may need to alter the vaccine content annually.

Data from international surveillance programs help experts predict which types in a given flu season are likely to circulate. Protection begins about 2 weeks after the vaccine is received.

Vaccinations for seasonal flu should begin in September, or as soon as the vaccine is ready. We keep going through the flu season, into January and beyond.

Flu shot side effects

The CDC states that there is a good safety record in the flu vaccine, and that it can not cause flu.

After having a vaccine, a person may experience the following adverse effects, but these will be mild and usually pass over within a few days.

  • pain, redness, and swelling at the injection site
  • headache
  • fever
  • nausea
  • muscle aches

About 1–2 people may develop a condition known as Guillain-Barr syndrome (GBS) in every 1 million. People may also develop GBS after having flu, however, and the risk of this is greater than with the vaccine. With the nasal spray version of the vaccine the risk of developing GBS may be lower.

If someone experiences hives, swelling, and breathing difficulties after any vaccine, they should seek immediate medical assistance, as this may be a sign of an allergic reaction. A serious reaction is called anaphylaxis which can endanger life.

People who have previously had an allergic reaction to a vaccine should not have the flu shot.

Flu shot pregnancy

Receiving the flu shot during pregnancy is healthy, and is recommended by doctors. Providing coverage takes about 2 weeks. The vaccine will also transfer to the fetus and give them some flu-protection.

Newborns can not have a flu vaccine but it can be unsafe for them to have flu. Both the mother and her unborn child can benefit from having the vaccine.

Flu shot effectiveness

The flu shot can not provide 100 per cent flu safety because scientists are unable to predict exactly what forms of flu will spread during a season.

CDC estimates from 2018–2019 indicate that the vaccine was nearly 47 percent effective in preventing influenza A or B.

Several factors can contribute to the effectiveness of each year’s flu shot. These may include the person receiving the vaccination’s age and overall health plus how well the vaccine suits the prevalent viruses.

Flu shot for seniors

The CDC is recommending a flu shot for people 65 or older. As people get older, when they have flu, they’re more likely to develop complications.

Older people may need a higher dose of vaccine because the flu virus may be less resistant to their immune systems. The vaccine is not capable of providing full protection but will reduce the risk of flu and complications.

During the 2013–2014 flu season in the U.S., a 2017 study looked at data for older people in hospital with flu. There were lower fatality rates, fewer complications and less overall time spent in an intensive care unit among those who had received the vaccine.

People 65 or older should ask their doctor about the vaccine at the beginning of each flu season. The doctor will prescribe an individually-adapted vaccine.

Flu shot cost

Within the private sector the cost of a flu shot is around $15–$24, depending on the type.

Individuals should check their insurance policies to find their cover. For example, Medicare part B provides for one flu shot each flu season.

When to see a doctor

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

  • they are already frail or have an existing health condition
  • they have a weakened immune system
  • they are infants or aged 65 years or over
  • their temperature remains high after 4–5 days
  • symptoms worsen or are severe
  • they become short of breath, develop chest pain, or both

However, anyone who has questions about their symptoms should get more advice from their doctor.


Is it really worth having the flu vaccine? It seems to me that it does not offer much protection.


Even though the flu vaccine is not 100% effective, there are still plenty of good reasons to get one every year.

First, getting the flu vaccine reduces the chances of becoming sick. In addition to being very unpleasant, flu can cause complications that can be potentially serious and even require hospitalization. Even when an infection is not serious, being sick with flu can also contribute to lost days of work or school.

Second, the vaccine may still protect a person even if they catch the flu. People that receive the vaccine and still get sick may have a less severe illness.

Lastly, by getting the flu vaccine, you’re protecting groups that cannot have the vaccination or who are susceptible to serious complications from the flu, such as older adults, children under 6 months of age, people with a weakened immune system, and individuals with chronic health conditions. Jill Seladi-Schulman, Ph.D.

Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.


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