Influenza (flu)

Overview

Influenza, usually called flu, is an infection of the airways caused by influenza viruses. Two types, influenza A and influenza B, cause the yearly outbreaks seen in people. The influenza A strains circulating at present are called H1N1 and H3N2. The virus passes from person to person in droplets from coughs and sneezes, and on hands and surfaces. It spreads quickly in crowded places such as schools, hostels and care homes.

Symptoms usually begin about two days after catching the virus and come on within hours. Fever, body aches, headache, deep tiredness and a dry cough are typical. Flu is not the same as a common cold, which starts slowly and mainly affects the nose and throat. It is also different from "stomach flu", which is a gut infection (gastroenteritis).

Most healthy people recover at home within about a week, although the cough and tiredness can last two weeks or longer. Flu can be dangerous for older people, young children, pregnant women and people with long-term illnesses or a weak immune system (the body's defence against infection). In these groups it can lead to pneumonia (lung infection), hospital admission and even death. Antiviral medicines can help people at higher risk, especially when started early. Antibiotics do not work against the flu virus. A flu vaccine every year is the best way to prevent it.

Symptoms

• Sudden fever or high temperature, sometimes with chills
• Dry cough, which can be severe and last two weeks or more
• Muscle and joint aches (myalgia)
• Headache, sometimes with sore eyes or discomfort in bright light
• Extreme tiredness and feeling very unwell (malaise)
• Sore throat
• Runny or blocked nose
• Poor sleep and loss of appetite
• Nausea, vomiting, diarrhoea or stomach pain, more often in children
• In children, ear pain and being less active than usual

Causes and risk factors

Causes

• Infection with an influenza virus, mainly influenza A (such as H1N1 or H3N2) or influenza B
• Breathing in droplets released when an infected person coughs, sneezes or talks nearby
• Touching a surface or hand with the virus on it and then touching your mouth, nose or eyes
• Close contact with an infected person, who can pass the virus on even before their own symptoms start and most easily in the first few days of illness

What makes it more likely

• Being aged 65 or over
• Children under 5 years, especially babies
• Pregnancy
• Long-term illness of the heart, lungs (such as asthma), kidneys or liver, diabetes, or a nervous system condition
• A weak immune system, for example from HIV, cancer treatment or long-term steroid medicines
• Working in health or social care, where contact with the virus is frequent
• Living or working in crowded places such as care homes, hostels or schools
• Not having had this year's flu vaccine

Complications

Most people have no lasting problems, but flu can make other illnesses worse. The most common complications are infections caused by bacteria taking hold after the virus, such as middle ear infection, sinus infection, bronchitis and pneumonia. Flu can also make long-term conditions like asthma, heart disease or diabetes harder to control. Less often, it causes inflammation (swelling and irritation) of the heart, brain or muscles. In severe cases, breathing can fail or the infection can spread through the blood (sepsis), and the person may need hospital care and help with breathing. Serious illness and deaths happen mostly in older people, young children, pregnant women and people with long-term health problems.

Diagnosis

During the flu season, doctors usually diagnose flu from the symptoms and an examination. Many other viruses, including COVID-19 and RSV (respiratory syncytial virus), cause a very similar illness, so symptoms alone cannot prove it is flu. Most people with mild illness do not need a test.

A test is useful when the result will change treatment or when there is an outbreak, for example in a hospital or care home. A swab is taken from the nose or throat. Rapid tests give a result in about 15 to 20 minutes but can miss some cases. A PCR test (which detects the virus genetic material) is more accurate but takes longer. For people at high risk or who are very ill, the doctor should not wait for a test result before starting antiviral treatment. A chest X-ray may be done if there are chest symptoms, to look for pneumonia.

Treatment

1. If you are otherwise healthy with mild flu, stay at home, rest, keep warm and drink plenty of fluids. Most people recover without needing to see a doctor.
2. Use paracetamol or ibuprofen to ease fever, headache and body aches. Follow the pack or your doctor or pharmacist for the dose, and do not take extra paracetamol alongside cold and flu remedies that already contain it.
3. Do not take antibiotics for flu. They do not kill viruses and will not help you get better faster. A doctor may prescribe them only if a bacterial infection such as pneumonia develops.
4. If you are pregnant, aged 65 or over, a young child, have a long-term illness or a weak immune system, or feel very unwell, see a doctor early. Antiviral medicines work best when started within one to two days of the first symptoms, but can still help people at high risk or in hospital when started later.
5. Antiviral medicines can shorten flu by about a day, make it milder and lower the risk of some complications. Your doctor decides which antiviral is suitable and the dose.
6. People with severe flu are treated in hospital, where they may need oxygen, help with breathing, and antiviral medicine given by mouth or through a drip.
7. After close contact with flu, or during an outbreak in a care home, a doctor may sometimes give an antiviral to people at high risk to help stop them catching it. This does not replace the yearly vaccine.

Medicines in Pakistan

Medicines a doctor may use, by group. Each link opens our page for that medicine, with every brand and its official DRAP price. Your doctor decides which medicine and dose are right for you.

Also used (not yet in our price list): Zanamivir (an inhaled flu antiviral; not recommended for people with asthma or COPD because it can cause wheezing), Peramivir (a flu antiviral given as a single drip into a vein), Baloxavir marboxil (a flu antiviral taken as a single dose by mouth), Antibiotics only when a doctor finds a bacterial infection such as pneumonia, ear infection or bronchitis on top of flu

Self-care and prevention

Self-care

• Rest, sleep and keep warm until you feel better
• Drink plenty of fluids such as water and warm drinks; your urine should be pale yellow or clear
• Take paracetamol or ibuprofen for fever and aches, and check labels so you do not take paracetamol twice in combined flu products
• Stay at home and keep away from others while you have a fever or feel too unwell for normal activities, especially in the first five days
• Cover your mouth and nose with a tissue, mask or handkerchief when you cough or sneeze, and throw tissues away straight after use
• Wash your hands often with soap and water
• Open windows to keep rooms well ventilated
• If you smoke, this is a good time to stop

Prevention

• Get a flu vaccine every year before the winter season; it is safe in pregnancy and is especially important for high-risk groups and health workers
• Wash your hands often with soap and water
• Cover coughs and sneezes and dispose of tissues properly
• Avoid close contact with people who are ill and avoid crowded places during the flu season
• Try not to touch your eyes, nose and mouth
• Keep living and working spaces well ventilated

Common questions

How can I tell flu from a common cold?

A cold comes on slowly, mainly affects the nose and throat, and most people can carry on with daily life. Flu starts suddenly, often within hours, with fever, body aches, headache and exhaustion that can keep you in bed. Only a lab test can confirm flu for certain, because other viruses can cause similar symptoms.

Do antibiotics help with flu?

No. Flu is caused by a virus, and antibiotics only work against bacteria. Taking them will not ease your symptoms or speed up recovery. A doctor may prescribe an antibiotic only if a bacterial infection, such as pneumonia, develops on top of flu.

How long is flu contagious?

You can pass flu on from shortly before your symptoms start, and you are most infectious in the first five days of illness. Staying at home while you have a fever, covering coughs and washing hands help protect others, particularly older people, babies and anyone with a long-term illness.

Why do I need a flu vaccine every year?

Flu viruses keep changing, and the protection from the vaccine fades over time, so a new vaccine is given each year. It takes about two weeks for protection to build up. Even when the vaccine does not stop flu completely, it reduces the chance of severe illness, complications and death.

Is the flu vaccine safe in pregnancy?

Yes. The injected flu vaccine can be given during pregnancy, and pregnant women are one of the main groups advised to have it every year because flu can be more serious for them. Speak to your doctor or midwife about getting it before the flu season.

Sources

1. WHO fact sheet. Influenza (seasonal) (2025).
2. CDC. Treating Flu with Antiviral Drugs | Influenza (Flu) | CDC (2026).
3. NICE technology appraisal guidance. 2 Clinical need and practice (Amantadine, oseltamivir and zanamivir for the treatment of influenza, TA168) (2009).
4. National Institute of Health (NIH) Islamabad, Pakistan. Seasonal Awareness and Alert Letter (SAAL), 61st Issue, Winter Season October 2024 - February 2025: Seasonal Influenza (2024).
5. NHS 111 Wales. NHS 111 Wales - Health A-Z : Flu (2025).
6. MedlinePlus. Flu | Flu Symptoms | Stomach Flu | Influenza | MedlinePlus (2024).
7. StatPearls (Boktor SW, Hafner JW, Doerr C). Influenza (Nursing) - StatPearls - NCBI Bookshelf (2023).

Prepared from the sources listed below by the MedInfoBase team. Last reviewed 16 October 2026. This page is general health information, not medical advice. It gives no doses and does not replace a doctor: talk to your doctor or pharmacist before starting, stopping or changing any medicine. How we write and check our pages.

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Common coldPneumoniaAcute bronchitisMiddle ear infection (otitis media)Sinusitis

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