Overview
Influenza, commonly called the flu, is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs, causing acute onset of fever, cough, sore throat, body aches, and fatigue.
Disease Summary
- Category
- Infectious Disease
- Cause
- Influenza viruses infecting the nose, throat, and sometimes the lungs
- Transmission
- Spreads mainly through respiratory droplets when an infected person coughs, sneezes, or talks, and through contact with contaminated surfaces
- Body System
- Respiratory System
- Preventable
- Yes
- Curable
- Yes
Symptoms
Common Symptoms
Flu symptoms usually begin suddenly and last around a week.
Sudden Fever
Fever typically begins abruptly, often one of the first noticeable symptoms.
Cough and Sore Throat
A dry cough and sore throat are common early symptoms.
Body Aches
Muscle and body aches are a hallmark feature that distinguishes flu from a common cold.
Fatigue
Extreme tiredness can begin suddenly and last for a week or more.
Danger Signs
Seek prompt medical attention if any of the following occur.
Difficulty Breathing
Shortness of breath or difficulty breathing needs urgent medical evaluation.
Chest or Abdominal Pain
Pain or pressure in the chest or abdomen can indicate a serious complication.
Confusion
Sudden confusion, particularly in older adults, is a warning sign requiring prompt care.
Symptoms That Improve Then Worsen
Flu symptoms that seem to get better and then suddenly return more severely may indicate a secondary infection.
Diagnosis
Clinical Assessment
Diagnosis is often based on symptoms and the pattern of sudden onset, especially during flu season.
Rapid Influenza Diagnostic Test
A quick test using a nose or throat swab can help confirm influenza infection.
RT-PCR Testing
A more sensitive laboratory test used to confirm influenza, particularly in more severe or uncertain cases.
Laboratory Tests
Rapid Influenza Diagnostic Test (RIDT)
Provides quick results but is less sensitive than laboratory-based testing.
RT-PCR Test
A highly sensitive test used to confirm influenza infection, particularly in hospitalized patients.
Chest X-ray
Used when pneumonia or another complication is suspected.
Treatment
Supportive Care
Rest, fluids, and over-the-counter medications to relieve fever and aches are the mainstay of treatment for most people.
Antiviral Medication
Recommended for people at higher risk of complications, most effective when started within 48 hours of symptom onset.
Prevention
Get an annual flu vaccine
Vaccination is the best way to protect against flu and reduce the risk of severe illness and hospitalization.
Wash hands regularly
Frequent handwashing helps reduce the spread of the virus.
Cover coughs and sneezes
Covering your mouth and nose when coughing or sneezing helps prevent spreading droplets to others.
Stay home when sick
Staying away from others while symptomatic helps prevent spreading the virus.
Risk Factors
High Risk
These groups face a substantially higher risk of severe influenza.
- Age under 5 or over 65
- Pregnancy
- Chronic medical conditions, such as heart, lung, or kidney disease
Increased Risk
These factors are associated with a moderately higher risk of complications.
- Weakened immune system
- Living in long-term care facilities
- Working in healthcare settings with frequent patient exposure
Additional Contributing Factors
Other factors linked with an increased likelihood of infection.
- Lack of annual vaccination
- Close contact with young children
- Crowded living or working environments
Complications
Pneumonia
One of the most common and serious complications of influenza.
Worsening of Chronic Conditions
Flu can worsen underlying conditions such as asthma, heart disease, or diabetes.
Secondary Bacterial Infections
Influenza can weaken the body’s defences, making bacterial infections of the ears, sinuses, or lungs more likely.
When to Seek Medical Care
Difficulty Breathing
Shortness of breath needs urgent medical evaluation.
Chest or Abdominal Pain
Pain or pressure in the chest or abdomen requires prompt assessment.
Confusion
Sudden confusion is a warning sign, especially in older adults.
Symptoms That Improve Then Worsen
May indicate a secondary infection and needs medical evaluation.
Prognosis
Good for Most Healthy People
Most people recover within about a week without lasting effects.
Higher Risk for Vulnerable Groups
Young children, older adults, and people with chronic conditions face a higher risk of severe illness.
Reduced Severity with Vaccination
Vaccinated people who still get the flu typically experience milder illness.
Differential Diagnosis
Common Cold
Usually causes milder symptoms that develop gradually, unlike the sudden onset typical of flu.
COVID-19
Shares many overlapping symptoms with influenza and often requires specific testing to distinguish.
Bacterial Pneumonia
Can occur on its own or as a complication of flu and may need distinct antibiotic treatment.
Strep Throat
Can cause fever and sore throat but typically lacks the body aches and cough common in flu.
References
World Health Organization (WHO)
WHO fact sheet on the symptoms, transmission, and prevention of seasonal influenza.
Centers for Disease Control and Prevention (CDC)
CDC guidance on flu symptoms, spread, and prevention.
Centers for Disease Control and Prevention (CDC)
CDC’s central resource for flu symptoms, vaccination, diagnosis, and treatment.
Frequently Asked Questions
Is the flu the same as a cold?
No. Flu symptoms usually begin more suddenly and are more severe than a cold, particularly body aches and fatigue.
How long is someone with the flu contagious?
People can be contagious about a day before symptoms start and for five to seven days after becoming sick, sometimes longer in children and people with weakened immune systems.
Do I need antiviral medication for the flu?
Not always. Most healthy people recover with rest and supportive care, but antivirals are recommended for people at higher risk of complications.
How effective is the flu vaccine?
Effectiveness varies by year and by how well the vaccine matches circulating strains, but vaccination consistently reduces the risk of severe illness and hospitalization.