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.
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
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
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
- 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
- muscle aches
- 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 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.
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.
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 symptoms are similar.
A cold and the flu both involve:
- a runny or blocked nose
- a sore throat
- a cough
- chest discomfort
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.
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
- 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:
- dry cough
- 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 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.
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.
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.
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 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
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, 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.
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.
The flu is not usually serious, but it is unpleasant. For some people, however, complications can arise. Some of these can be life threatening.
- bacterial pneumonia
- 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
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.
- 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
- 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.
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%.
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.
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 concerns about their symptoms should speak to their doctor for further advice.