Reviewed by The Wellness Team. Last reviewed October 2026.
Flu arrives fast: one hour you feel fine, the next you have a fever, aching muscles and no energy for anything. Most healthy people recover at home in about a week, but some people need treatment and some symptoms should never be watched and waited on. Here is how to tell the difference.
What flu is
Flu is influenza, a viral infection of the nose, throat and lungs that circulates mainly between October and March in the UK. It spreads through coughs, sneezes and contaminated surfaces, and it is at its most contagious in the first few days of symptoms. It is not the same illness as a heavy cold, and the difference matters when deciding what to do.
Flu symptoms
Sudden onset: symptoms appear over hours, not days
Fever of 38C or above, with chills and sweating
Aching muscles, especially in the back, arms and legs
Pronounced exhaustion, often bad enough to keep you in bed
Dry cough, sore throat and headache
Sometimes a blocked or runny nose, and in children, earache, nausea or vomiting
A cold builds gradually, stays above the neck, rarely causes a high fever, and leaves you able to function. Flu floors you. That sudden, whole body severity is the single most useful clue.
How long flu actually lasts
The fever and worst aches usually settle within three to five days, and most people are substantially better within a week. The cough and tiredness can linger for two to three weeks, which is normal and not, on its own, a reason to worry. You are generally infectious from the day before symptoms start until around five days after, longer in children.
What actually helps
Rest, fluids and paracetamol or ibuprofen for the fever and aches are the mainstay. Antibiotics do nothing for flu because it is viral. Antiviral medicines such as oseltamivir are reserved by NICE guidance for specific situations, mainly people at higher risk of complications, and they only work if started within 48 hours of symptoms beginning, which is why timing matters if you are in an at risk group.
Who should speak to a doctor about flu
Most healthy adults can manage flu at home. NICE guidance recommends medical advice for people at higher risk of complications, because antiviral treatment or closer monitoring may be appropriate:
Anyone aged 65 or over
Pregnant women
People with long term conditions: asthma, COPD, diabetes, heart, kidney or liver disease
Anyone with a weakened immune system
Babies and young children who are feeding poorly, listless, or struggling with breathing
When flu is urgent
Seek urgent medical help if any of the following apply:
Breathlessness, chest pain, or coughing up blood
Confusion, severe drowsiness, or difficulty waking
Signs of dehydration: passing little urine, dizziness on standing
Symptoms improving and then suddenly worsening with a new high fever
The last pattern matters particularly: it can signal a secondary bacterial chest infection or pneumonia, which does need antibiotics.
What an online GP can do today
A GP can assess your symptoms by video or phone the same day, advise on treatment, and prescribe antivirals where clinically appropriate if you are within the treatment window. If the picture suggests a secondary infection, the GP can prescribe antibiotics or arrange the right next step, and a fit note can be issued where needed so work is one less thing to carry.
Appointments cost £59 by phone, £150 by video and £220 in person at 10 Portman Square, Marylebone, with the price confirmed before you book. Same day appointments are usually available. Message us on WhatsApp: https://wa.me/447961280835
Frequently asked questions
How long is flu contagious?
From about a day before symptoms appear until around five days after, and longer in children. Staying home while you have a fever protects the people around you who are most vulnerable.
Do antibiotics help flu?
No. Flu is a virus, so antibiotics have no effect on it. They become relevant only if a secondary bacterial infection develops, which is one of the things a GP assessment looks for.
Flu or a bad cold: how can I tell?
Speed and severity. A cold builds over a couple of days and stays mild; flu arrives within hours, brings a real fever and whole body aches, and usually puts you in bed. If you are unsure, a short consultation settles it.
Can I get a sick note for flu?
Yes. Where flu keeps you off work beyond self certification, a GP can issue a fit note after assessing you, including by phone or video.
Sources: NICE Clinical Knowledge Summaries, Influenza (seasonal); UK Health Security Agency guidance on seasonal influenza.
Reviewed by The Wellness Team