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Influenza, or flu, is a respiratory illness that results from a viral infection. Flu is highly contagious and spreads through respiratory droplets. A person can pass it on while talking or through physical contact, such as shaking hands.

Influenza A and influenza B cause seasonal epidemics in the United States and elsewhere every winter. Type C usually causes mild respiratory illness.

Some strains of influenza A, such as the H5N1 "bird flu" virus, occasionally infect humans, causing serious illness. Experts track these strains carefully, as they try to predict how they will change, and how they might affect people.

In this article, we explain the symptoms of flu, the treatment options, how it differs from a cold, and how to prevent the flu.

a woman sat in bed and blowing her nose because she has the flu. Share on Pinterest
A person with the flu may experience a stuffy or runny nose.

According to the Centers for Disease Control and Prevention (CDC), a person with flu may experience:

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

Not everyone with flu will have all of these symptoms. For instance, it is possible to have flu without a fever.

The symptoms of influenza typically come on suddenly. Initially, a person with flu may 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

Why do people sometimes have chills but no fever?

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
  • a 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 can also have gastrointestinal symptoms, such as nausea, vomiting, and diarrhea.

If a child has the following symptoms, they need emergency medical care:

  • 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

What happens when a baby gets a cold?

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

Learn more here about influenza A.

Flu type B symptoms

Influenza B symptoms are similar to those of influenza A.

Learn more here about influenza B.

Most people will be able to treat the flu at home. A combination of lifestyle remedies and over-the-counter medication can help relieve symptoms.

Pain relief medication can help manage a headache and body pains. A healthcare professional can recommend the best options.

Some painkillers, such as aspirin, are not suitable for children under 16 years of age. The use of aspirin at this age can lead to a condition known as Reye's syndrome.

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

How long does flu last? Find out here.

Flu medication

A virus causes flu, so antibiotics will not cure the illness. A doctor will only prescribe antibiotics if a bacterial infection is present alongside the flu. However, antiviral medications may help when someone has the flu.

Here are some tips on how to treat a cold or flu at home.

Antivirals aim to stop the virus from multiplying in a person's body. Examples include oseltamivir (Tamiflu) and zanamivir (Relenza).

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

People can receive this treatment if they are aged 12 years or more and have had symptoms for fewer than 48 hours. Possible side effects include diarrhea and bronchitis.

Some research shows that flu medication may affect a person's heart. Find out more here.

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

However, there is not enough evidence available to confirm that consuming these helps.

Which foods are a good choice for a person with flu?

If a person seeks medical advice for flu symptoms, a doctor will likely ask about their symptoms and do a physical examination. A doctor may also take a throat swab for testing.

The rapid influenza diagnostic test can produce results in 10–15 minutes but may not be accurate. Other, more accurate tests can take longer to give results.

Flu or a cold?

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Image credit: Stephen Kelly, 2018.

People often confuse the flu with a bad cold, as some symptoms are similar.

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?

There are many types of viruses, and some can affect the digestive system. People sometimes call this "stomach flu." This illness is different from influenza, which is a respiratory disease.

The most common cause of "stomach flu" is the norovirus, which enters the body through contaminated food or drink. Symptoms include nausea, vomiting, and diarrhea. Food poisoning causes similar symptoms.

Is it a stomach virus or food poisoning? Find out here.

Flu or pneumonia?

Pneumonia can be bacterial or viral. The symptoms can resemble those of flu, but an individual may have a sharp, stabbing pain in the chest, especially when they breathe deeply or cough.

Bacterial pneumonia can start slowly or suddenly. Symptoms can include:

  • 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

However, unlike the flu, symptoms of pneumonia usually develop gradually. Anyone who has a high fever and breathing difficulties should see a doctor immediately.

What is viral pneumonia? Find out here.

People can have the flu at any time, but it is more common during the flu season. The timing and duration of the flu season change from year to year, but it usually happens around fall and winter.

Flu activity often starts to increase in October, and it can last as late as May. However, it is most common from December to 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 also stay away from others when they have the flu to avoid spreading it themselves.

Click here for more information on how to avoid flu.

The flu virus transmits through droplets of liquid. A person can pass the virus on to another person who is up to 6 feet away from them when they cough, sneeze, talk, or breathe.

A healthy individual can pass on the virus a day before they, themselves, have symptoms. In other words, it is possible to pass on the flu before you know you have it. The infected individual can continue to transmit the virus for up to 5–7 days after symptoms appear.

People with a weakened immune system, older people, and young children may be able to pass on the virus for longer than this.

Flu is most contagious in the first 3–4 days after symptoms appear.

How long is flu contagious? Find out here.

A person can develop flu symptoms if droplets that contain the virus and come from the breath of another person enter their mouth, nose, or lungs.

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.

Research shows that just breathing can spread the flu virus. Learn more here.

The incubation period of a disease is the time it takes from when the virus infects a person to when the symptoms start.

For flu, this is around 2 days, but it can vary from 1 to 4 days.

A person can transmit the virus even before symptoms appear.

Flu can be more serious during pregnancy, as pregnancy affects how the immune system works. If a woman is pregnant and has flu, they may need to spend time in the hospital.

Pregnancy-related complications include a higher risk of:

  • preterm birth
  • low birth weight
  • stillbirth

Flu can be fatal for newborns. Risks to the mother include a greater chance of having complications, such as bronchitis, ear and blood infections.

What are some other causes of fever during pregnancy? Find out here.

Flu symptoms appear suddenly, usually around 2 days after infection. Most symptoms disappear after about 1 week, 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 their symptoms have disappeared.

If complications develop, these can take longer to resolve. Some of the more severe types of complications can have a long term impact on a person's health, for example, kidney failure.

Some people experience post-viral fatigue for a week or so after the main symptoms disappear. They may have a lingering sense of tiredness and feeling unwell.

Learn more here about post-viral syndrome and how to manage it.

How long does it take for a cold to pass? Find out here.

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.

How long does the flu last? Find out here.

The flu is not usually serious, but it is unpleasant. For some people, however, complications can arise. Some of these can 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. What is bird flu, and should we worry about it? Find out here.

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.

What does a flu shot contain?

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

However, viruses adapt and change over time, and scientists may need to adjust the content of the vaccines each year.

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

Seasonal flu vaccinations should start in September or as soon as the vaccine is ready. They continue throughout the flu season, into January and beyond.

Flu shot side effects

The CDC note that the flu vaccine has a good safety record, and it cannot cause flu.

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

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

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

If someone experiences hives, swelling, and difficulty breathing after any vaccine, they should seek immediate medical help as these may be the sign of an allergic reaction. A severe reaction is known as anaphylaxis, which can be life threatening.

Click here to find out how to recognize anaphylaxis and what to do if it happens.

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

Is the flu shot safe? Find out here.

Flu shot pregnancy

It is safe to have the flu shot during pregnancy, and doctors recommend it. It takes about 2 weeks to provide protection. The vaccine will also pass through to the fetus and give them some protection from the flu.

Newborns cannot have a flu vaccine, but having flu can be dangerous for them. Having the vaccine can benefit both the mother and her unborn child.

Flu shot effectiveness

The flu shot cannot offer 100% protection from the flu as scientists cannot predict precisely what flu types will circulate during a season.

CDC figures from 2018–2019 show that the effectiveness of the vaccine in preventing influenza A or B was about 47%.

Many factors can contribute to the effectiveness of the flu shot from year to year. These can include the age and overall health of the person receiving the vaccination, plus how well the vaccine matches the prevalent viruses.

Flu shot for seniors

The CDC recommend a flu shot for people aged 65 years or more. As people get older, they are more likely to develop complications if they have flu.

Older people may need a higher dose of vaccine, as their immune systems may be less able to resist the flu virus. The vaccine cannot provide complete protection but will reduce the risk of flu and the severity of complications.

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

People who are 65 years of age or older should ask their doctor about the vaccine at the start of each flu season. The doctor will recommend a vaccine that suits the individual.

Flu shot cost

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

People should check their insurance policies to find out their cover. Medicare part B, for example, provide for one flu shot every flu season.

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 concerns about their symptoms should speak to their doctor for further advice.

How can you diagnose a fever at home? Find out here.

Q:

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

A:

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.