Flu (influenza)
Also known as: flu, influenza, seasonal flu, the flu
Flu is a respiratory illness caused by influenza viruses.
When to seek care
Call 999 or go to A&E for sudden chest pain, difficulty breathing or coughing up a lot of blood. An urgent GP appointment or NHS 111 is advised when someone with flu symptoms is 65 or over, pregnant, has a long-term condition or a weakened immune system, or has not improved after 7 days. The CDC says most people with flu have a mild illness and do not need medical care.
Overview
Flu is an infection caused by influenza viruses. It comes on very quickly, with a sudden high temperature, aching body, tiredness, dry cough, sore throat and headache. Most people have a mild illness, but flu can lead to complications such as bronchitis and pneumonia, and older adults, pregnant women, young children and people with long-term conditions are at higher risk. The NHS says having the vaccine before the flu season starts is the best way to avoid catching and spreading flu.
Causes
- Infection with influenza viruses; types A and B cause the seasonal epidemics seen in people.
- The virus spreads mainly in droplets made when people with flu cough, sneeze or talk.
- The WHO says the virus can also be spread by hands contaminated with influenza viruses.
- NHS inform says germs from coughs and sneezes can survive on hands and surfaces for 24 hours.
Risk factors
- Older age
- Pregnancy
- Young children
- Long-term conditions such as asthma, diabetes or heart disease
- A weakened immune system
Symptoms
What a clinician may find
- A sudden high temperature, aching body and dry cough
Complications
- Bronchitis
- Pneumonia
- Sinus and ear infections
- Inflammation of the heart or brain
- Worsening of long-term conditions such as asthma and heart disease
- In some cases, treatment in hospital
Diagnosis
Flu can be confirmed with a test on a swab from the nose or throat. MedlinePlus describes rapid antigen tests as the most common flu tests but says they are not as accurate as other types. It describes molecular laboratory tests as the most accurate type of flu test.
Treatment
Most people recover at home with rest, plenty of fluids and paracetamol or ibuprofen to lower a temperature and ease aches. GPs do not recommend antibiotics for flu, because it is caused by viruses. Antiviral medicines are prescription-only; the CDC recommends prompt treatment for people at higher risk of complications and says they work best when started within about two days. The NHS says aspirin should not be given to children under 16.
Prevention
The NHS says having the vaccine before the flu season starts is the best way to avoid catching and spreading flu. NHS advice also includes washing hands often with warm water and soap, binning used tissues quickly, and covering the mouth and nose when coughing or sneezing.
Key facts
| Fact | Detail |
|---|---|
| Also called | Flu, influenza, seasonal flu |
| Caused by | Influenza viruses, mainly types A and B in seasonal epidemics |
| Spreads mainly by | Droplets from coughing, sneezing and talking |
| Onset | Very quick, with a sudden high temperature |
| Confirmed by | A test on a nose or throat swab |
| Usual care | Rest, fluids and paracetamol or ibuprofen at home; antibiotics are not recommended |
| Main protection | Vaccination before the flu season, plus regular handwashing |
What are the symptoms of flu, and how is it different from a cold?
The NHS lists the main symptoms of flu as a sudden high temperature, an aching body, tiredness, a dry cough, a sore throat and a headache. Flu comes on very quickly. In children the symptoms are similar, but they can also have ear pain, sore eyes and swollen glands, and may seem less active, according to NHS England.
The CDC compares the two illnesses side by side. Flu typically starts abruptly, while cold symptoms build gradually. Tiredness and weakness are usual with flu but only sometimes happen with a cold.
Most people feel better within about a week, according to the CDC, although a cough and tiredness can last for a few more weeks. The CDC also says most people with flu have a mild illness and do not need medical care.
How does flu spread?
Flu is caused by influenza viruses. The CDC says there are four types, but influenza A and B viruses are the ones that circulate and cause seasonal epidemics. Type A viruses are grouped into subtypes, and A(H1N1) and A(H3N2) are currently circulating in humans.
The CDC says most experts believe flu spreads mainly through droplets made when people with the infection cough, sneeze or talk. The WHO adds that the virus can also be spread by hands contaminated with influenza viruses. NHS inform says germs from coughs and sneezes can survive on hands and surfaces for 24 hours.
The CDC says the incubation period is about two days, ranging from one to four days. People are most contagious during the first three days of illness, and young children and people with weakened immune systems may be contagious for longer.
How is flu diagnosed?
Flu can be confirmed with a rapid or laboratory test on a swab. MedlinePlus describes a standard test in which a clinician rotates a nasopharyngeal swab for 10 to 15 seconds before the sample is tested for influenza A and B. The CDC says rapid tests can use nasopharyngeal, nasal or throat swabs.
MedlinePlus says rapid antigen tests are the most common flu tests but are not as accurate as other types. Rapid molecular tests can give results quickly, and molecular laboratory tests are the most accurate type of flu test, although results can take up to three days.
The CDC also says that most people with flu have a mild illness and do not need medical care or antiviral drugs.
What is the treatment for flu?
Most people recover at home. NHS sources describe rest, sleep, warmth and plenty of water to avoid dehydration, with paracetamol or ibuprofen to lower a temperature and ease aches and pains. The NHS says aspirin should not be given to children under 16, and its Reye's syndrome page links that serious condition to aspirin use in this age group.
GPs do not recommend antibiotics for flu. Flu is caused by viruses, and antibiotics will not relieve symptoms or speed up recovery.
Antiviral medicines such as oseltamivir and zanamivir are available on prescription only. An NHS Borders leaflet says most people will not need them because their illness is mild and they recover quickly. The CDC recommends prompt treatment for people at higher risk of serious complications. It says antivirals work best when started within about two days, although a later start may still help people at higher risk or who are very sick.
What are the complications of flu, and when should someone get medical help?
The CDC lists pneumonia, bronchitis, sinus infections and ear infections among the complications of flu. It also lists inflammation of the heart and brain, and says flu can make chronic conditions such as asthma and heart disease worse. One NHS hospital page notes that some cases need treatment in hospital.
The CDC names young children, adults aged 65 and over, pregnant women and people with conditions such as asthma, chronic lung disease, diabetes or heart disease as being at higher risk of serious flu complications.
NHS England advises calling 999 or going to A&E for sudden chest pain, difficulty breathing or coughing up a lot of blood. It advises an urgent GP appointment or NHS 111 when someone with flu symptoms is 65 or over, pregnant, has a long-term condition or a weakened immune system, or has symptoms that do not improve after 7 days.
Who is offered the flu vaccine, and what else helps prevent flu?
The NHS says having the vaccine before the flu season starts is the best way to avoid catching and spreading flu. Protection is not guaranteed, but any flu after vaccination is typically milder, according to the NHS vaccine page.
NHS England says the free vaccine is offered to adults aged 65 and over, adults aged 18 to 64 with certain long-term health conditions, and pregnant women.
UKHSA's guide to the autumn 2026 programme also lists children, residents of long-stay care homes, close contacts of people with a weakened immune system, carers on Carer's Allowance and some frontline social care workers. The clinical risk groups include chronic respiratory disease, chronic heart disease, chronic liver disease, diabetes and a weakened immune system. Start dates differ between groups, so the UKHSA guide and NHS pages hold the current details.
Frequently asked questions
Is flu the same as a cold?
They are different illnesses. The CDC says flu typically starts abruptly, while cold symptoms build gradually. It also says tiredness and weakness are usual with flu but only sometimes happen with a cold. The NHS lists the main flu symptoms as a sudden high temperature, an aching body, tiredness, a dry cough, a sore throat and a headache.
Do antibiotics work for flu?
No. GPs do not recommend antibiotics for flu, because flu is caused by viruses and antibiotics will not relieve symptoms or speed recovery. Antiviral medicines are prescription-only; the CDC recommends prompt treatment for people at higher risk and says they work best when started within about two days. Most people recover with rest, fluids and simple pain relief.
How long is someone contagious with flu?
The CDC says people with flu are most contagious during the first three days of illness, and the incubation period is about two days, ranging from one to four days. Young children and people with weakened immune systems may be contagious for longer. NHS England says people are most likely to pass flu on in the first five days.
Who is most at risk of serious illness from flu?
The CDC names young children, people aged 65 and over, pregnant women and people with conditions such as asthma, chronic lung disease, diabetes or heart disease as being at higher risk of serious flu complications. People with a weakened immune system are also included in UKHSA's clinical risk groups for the flu vaccine.
When does flu need emergency help?
NHS England advises calling 999 or going to A&E for sudden chest pain, difficulty breathing or coughing up a lot of blood. An urgent GP appointment or NHS 111 is advised for people aged 65 or over, who are pregnant, or who have a long-term condition or weakened immune system, and for anyone whose symptoms have not improved after 7 days.
Connected pages in detail
What each linked page on Anatomy Nexus covers, in a line, for Flu (influenza).
Conditions
- Pneumonia Infection of the alveoli, which fill with fluid and inflammatory cells so that part of the lung cannot exchange gas.
- Asthma A chronic inflammatory disease of the airways causing reversible narrowing, with episodes of wheeze, breathlessness, chest tightness and cough.
- Chronic obstructive pulmonary disease (COPD) Progressive, largely irreversible airflow obstruction caused by damage to the airways (chronic bronchitis) and destruction of alveoli (emphysema).
Symptoms
- Fever A body temperature of 38 °C (100.4 °F) or more, produced when the hypothalamus raises its set point in response to inflammatory signals.
- Cough A reflex blast of air that clears the airways of mucus, irritants and foreign material.
- Fatigue Persistent tiredness that rest does not fix.
- Headache Pain felt anywhere in the head.
Medical tests
- Microbiology tests Tests that find the organism causing an infection: growing it on culture, detecting its genes by PCR, its proteins by antigen tests.
Medications
- Influenza vaccine The yearly flu vaccine: an inactivated or live attenuated vaccine reformulated each season to match circulating influenza strains.
Guides and news
- MHRA updates Fluenz fainting information The MHRA's September 2026 Safety Roundup says Fluenz nasal flu vaccine information now notes a higher chance of fainting soon after the spray is…
- Childhood vaccinations: the NHS schedule The NHS offers babies and children vaccines from 8 weeks old through school, and a GP surgery is the contact for any dose that was missed.