The Flu (Influenza): Everything You Need to Know

Influenza, or flu, is a respiratory illness that results from a viral infection. The flu is highly contagious and spreads thru respiratory droplets. A person can pass it on while talking or thru physical contacts, 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.

Symptoms

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

  • 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 the flu may trust the source of experience. trusted source:

  • 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
  • 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

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.

For more information and resources to help keep you and your loved ones healthy this flu season, visit our dedicated hub.

Treatment

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.

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.

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 AdministrationTrusted Source (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.

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: trusting a source

  • chicken broth
  • herbal teas
  • vitamin supplements

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

Flu diagnosis

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?

People often confuse the flu with a bad cold, as some symptomsTrusted Source 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 usually cause a temperature, although the flu does.
  • A cold’s symptoms usually appear gradually, however flu symptoms might appear suddenly.
  • Cold symptoms are usually milder than flu symptoms.
  • A person who has the flu may feel fatigued for several weeks thereafter.
  • The flu is more likely to cause problems, and it can be fatal.

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.

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.

When is flu season?

People can have the flu at any time, but it is more common during the flu season. The timing and durationTrusted Source of the flu season changes 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.

Prevention

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.

Contagiousness

The flu virus transmits thru 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.

Transmission

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 thru a trusted source 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.

Incubation period

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, according to a trusted source, but it can vary from 1 to 4 days.

A person can transmit the virus even before symptoms appear.

Flu when pregnant

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.

How long does it last?

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, it can take longer to resolve them. 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 feel unwell.

Timeline

Typically, flu may progress as follows:

  • A person is infected by the virus through their nose or mouth.
  • They may be able to spread the virus to others after one day.
  • 1–2 days after infection, symptoms begin.
  • The virus is most likely to be transmitted 3–4 days after symptoms develop.
  • The fever and muscle aches subside after four days.
  • The majority of symptoms go away after a week.
  • The risk of spreading the virus vanishes 5–7 days after the first symptoms show.
  • The cough and exhaustion could last another week.

Precautions

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

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 virusesTrusted Source, 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 noteTrusted Source 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

A condition known as Guillain-Barr syndrome (GBS) may affect 1–2 people in every 1 million. However, people can also develop GBS after having the 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 signs 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.

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%Trusted Source.

Many factors (trusted source) 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, as well as how well the vaccine matches the prevalent viruses.

Flu shot for seniors

For those aged 65 and up, the CDC recommends getting a flu shot. When people catch the flu, they are more likely to suffer complications as they get older.

Because their immune systems may be less able to combat the flu virus, older persons may require a higher dose of vaccine. Although the vaccine does not guarantee total protection, it does lessen the risk of flu and the severity of its effects.

During the 2013–2014 flu season in the United States, Trusted Source examined data on elderly adults admitted to hospitals with the flu. There were fewer fatalities, fewer problems, and less overall time spent in an intensive care unit among those who had received the vaccine.

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 what they cover. Medicare part B, for example, provides for one flu shot every flu season.

When to see a doctor

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

  • If they are already frail or have an existing health condition,
  • They have a weakened immune system.
  • If they are infants or aged 65 years or over,
  • Their temperature remains high after 4–5 days.
  • Symptoms worsen or are severe when
  • 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.

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