Overview
Influenza (flu) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness and can lead to hospitalisation or death, particularly in vulnerable groups. Flu is more severe than the common cold, typically causing a sudden onset of symptoms. Annual flu vaccination is the most effective prevention. Epidemics occur each year in winter, and occasionally pandemics arise from novel influenza strains.
Causes and risk factors
The following factors are associated with the development of Influenza (Flu):
- Influenza A and B viruses
- Spread through respiratory droplets when infected person coughs, sneezes, or talks
- Contact with contaminated surfaces then touching mouth, nose, or eyes
- Close contact in crowded settings
- Virus changes each year (antigenic drift) requiring annual vaccination
- High-risk groups: elderly, young children, pregnant women, immunocompromised
Signs and symptoms
Common signs and symptoms of Influenza (Flu) include:
- Sudden onset of fever and chills
- Muscle aches and pains
- Headache
- Severe fatigue
- Dry cough
- Sore throat
- Runny or stuffy nose
- Some people have vomiting and diarrhoea
- Symptoms usually last 1–2 weeks
- Fatigue may persist longer
Diagnosis
Influenza (Flu) is typically diagnosed through a combination of medical history, physical examination, and appropriate investigations such as blood tests, imaging, or specialist review, depending on the specific condition and presentation. Your doctor will consider all relevant symptoms and risk factors before making a diagnosis.
Treatment options
Treatment approaches for Influenza (Flu) may include:
- Rest and adequate fluid intake
- Paracetamol or ibuprofen for fever and pain
- Antiviral medications (oseltamivir/Tamiflu) for high-risk groups if started within 48 hours
- Hospitalisation for severe cases or complications
- Annual flu vaccination for prevention
- Avoiding contact with others while unwell
Prevention
Annual flu vaccination — most effective prevention. Hand hygiene, avoiding close contact with sick individuals, staying home when unwell.
Complications
If left untreated or poorly managed, Influenza (Flu) can lead to complications including:
Pneumonia, bronchitis, myocarditis, encephalitis, and worsening of chronic conditions. Flu causes 290,000–650,000 deaths annually worldwide.
Sources and references: This article is written in accordance with NHS UK, WHO, and Mayo Clinic clinical guidelines. It is reviewed for accuracy by the SymptomSense content team. ICD-10 code: J10. Last reviewed June 2026.