Influenza (Flu)

Influenza is a common viral infection that can lead to serious complications, especially in high-risk groups such as young children, older adults, and people with certain chronic health conditions. Every year, influenza epidemics cause millions of cases worldwide, resulting in numerous hospitalisations and deaths. Understanding the causes, symptoms, and prevention methods of influenza is essential for effectively managing this disease.

What is Influenza?

Influenza, commonly referred to as the flu, is a contagious respiratory illness caused by influenza viruses. These viruses primarily infect the nose, throat, and lungs, causing a range of symptoms, including fever, cough, sore throat, runny nose, body aches, headaches, and fatigue. 

Influenza can affect people of all ages but is particularly dangerous for young children, older adults, and individuals with weakened immune systems. The severity of influenza can range from mild to severe and may sometimes lead to complications, such as pneumonia, which can be life-threatening.

According to the CDC (2025), there are four types of influenza viruses: 

• Influenza A: This type of influenza virus is the most common and can infect humans and animals. Influenza A viruses are further classified into subtypes based on the proteins on their surface: haemagglutinin (H) and neuraminidase (N). Examples include:
  • H1N1: This subtype is commonly referred to as "swine flu" because it was initially identified in pigs. H1N1 can cause mild to severe respiratory illness and has been responsible for seasonal flu outbreaks.
  • H3N2: This subtype is known for causing seasonal flu epidemics. It often undergoes changes and mutations, leading to variations that can affect vaccine effectiveness each year.
• Influenza B: Influenza B viruses are only found in humans and cause seasonal outbreaks. They are not classified into subtypes but can be broken down into lineages and strains. Influenza B generally causes milder symptoms compared to influenza A.
• Influenza C: This type of influenza virus is less common and usually causes mild respiratory illness. It does not cause epidemics.
• Influenza D: Influenza D viruses primarily affect cattle and are not known to infect or cause illness in humans.

Understanding the different types of influenza is essential for developing effective vaccines and treatment strategies.

The early signs of flu include:

• Sudden onset of fever  
• Chills  
• Fatigue  
• Dry cough  
• Sore throat  
• Runny or stuffy nose  
• Muscle aches  
• Headaches  
• Nausea or diarrhoea
• Weakness and malaise  

If you notice these early signs of influenza, it's essential to monitor your symptoms and seek medical advice if necessary.

The symptoms of influenza can vary in severity and typically appear suddenly. The WHO (2025) states that symptoms of flu typically appear 1-4 days after infection and usually last around a week.  Some of the most common symptoms include:

• Fever: A high fever, typically above 100.4°F (38°C), is one of the most common symptoms of influenza. Unlike a cold, the fever associated with flu tends to last for 3 to 4 days.
• Chills: Chills often accompany the fever in influenza cases. While chills are rare with a cold, they are a frequent symptom of the flu.
• Body aches: Influenza can cause severe body aches, particularly in the head, lower back, and legs. These aches are usually more intense than the mild ones experienced with a cold.
• Fatigue: Extreme weakness and tiredness are hallmark symptoms of the flu. This fatigue can be debilitating, interfering with daily activities and requiring ample rest for recovery.
• Cough: A dry, persistent cough is another common symptom of influenza. The cough can be severe, leading to chest tightness and wheezing.
• Sore throat: Influenza can cause a scratchy, irritated sore throat that may worsen as the viral infection progresses. This symptom is also common in colds.
• Runny or stuffy nose: While more commonly associated with colds, a runny or stuffy nose can also occur with the flu.
• Headache: Headaches are more likely to occur with influenza and can be quite severe, unlike the milder headaches sometimes experienced with colds.
• Eye pain: Red, watery eyes and eye pain can be symptoms of the flu, though these are less common than other symptoms.
• Loss of appetite: Many people with influenza experience a loss of appetite, which can contribute to the fatigue and weakness associated with the illness.

Influenza typically progresses through several distinct stages, each characterised by specific symptoms and responses in the body. These include:

• Exposure to the virus: Influenza viruses spread mainly through tiny droplets made when people with flu cough, sneeze, or talk. You can inhale these droplets directly or pick up the germs from touching an object and then touching your eyes, nose, or mouth. The time from when a person is exposed to and infected with the influenza virus to when symptoms begin is approximately 1 to 4 days, with a median of 2 days.
• Day 1: Symptoms of influenza can include sudden onset of fever, cough (usually dry), headache, muscle and joint pain, severe malaise (feeling unwell), sore throat, and runny nose.
• Days 2 to 3: During these days, the symptoms are at their worst. People with the flu may experience symptoms such as fever or feeling feverish/chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, fatigue (tiredness), and some individuals may also experience vomiting and diarrhoea. However, this is more common in children than in adults.
• Days 4 to 5: Most people who contract the flu typically recover within 7 to 10 days without requiring medical treatment. Rest and drinking plenty of liquids are recommended.
• Day 6 and beyond: Recovery from the flu typically takes about a week, but fatigue and weakness may persist for up to three weeks. It's essential to resume activities gradually to avoid overexertion.

Knowing the different stages of influenza can help individuals take appropriate measures to manage their symptoms and prevent the spread of the virus to others.

Influenza is primarily caused by the influenza virus, which has several types and strains that can lead to outbreaks. The virus spreads through droplets in the air when an infected person coughs, sneezes, or talks. You can inhale these droplets directly or contract the virus by touching contaminated surfaces and then touching your eyes, nose, or mouth. 

• Influenza virus: Type A is known for causing most flu pandemics and can infect both humans and animals. Type B typically leads to seasonal outbreaks that primarily affect humans. Type C usually causes mild respiratory illness and does not result in epidemics. Type D primarily affects cattle and is not known to infect humans.
• Transmission: Influenza viruses spread easily from person to person through respiratory droplets when an infected person coughs, sneezes, or talks. It can also spread by touching contaminated surfaces and then touching the face.
• Environmental factors: Flu season typically occurs in colder months when people are more likely to be indoors, facilitating virus transmission. Dry air can also help the virus survive longer outside the body.
• Human behaviour: Close contact with infected individuals, crowded places, and poor hygiene practices increases the risk of infection.
• Compromised immune systems: Individuals with weakened immune systems, such as older adults, young children, and those with chronic health conditions, are more susceptible to infection.

Understanding these causes can help in taking preventive measures to reduce the risk of influenza infection.

Certain factors can increase the likelihood of contracting influenza, including:

• Age (children under five and adults over 65)
• Chronic medical conditions (asthma, diabetes, heart disease)
• Weakened immune system
• Pregnancy
• Obesity
• Certain medications (corticosteroids, chemotherapy)
• Occupational exposure (healthcare workers, caregivers)

Who's at Risk?

Certain groups of people are at a higher risk of experiencing severe complications from influenza, including:

• Young children, especially those under 5 years old
• Adults aged 65 and older
• Pregnant women
• People with chronic medical conditions
• Residents of nursing homes and long-term care facilities
• People with weakened immune systems (due to disease or medication)
• Healthcare workers and caregivers

Understanding the risk factors and who is most susceptible to influenza can help individuals take proactive measures to prevent infection and seek prompt medical care if symptoms develop.

Most people with the flu recover on their own; however, influenza can sometimes lead to severe, and potentially life-threatening complications. These may include:

• Pneumonia: Pneumonia is one of the most serious complications of influenza, which can be caused by the flu virus itself or a secondary bacterial infection. This condition may lead to acute respiratory distress syndrome (ARDS) and other critical respiratory issues, requiring hospitalisation and intensive care.
• Bronchitis: Bronchitis, an inflammation of the bronchial tubes, is another respiratory complication associated with influenza. It can cause persistent coughing, mucus production, and difficulty breathing, leading to further respiratory distress.
  • Ear and sinus infections: Influenza can also lead to ear and sinus infections, particularly in children. Middle ear infections and sinusitis (inflammation of the sinuses) are common complications that can cause pain and discomfort, necessitating medical treatment.
• Cardiac complications: Cardiac complications linked to influenza include myocarditis (inflammation of the heart muscle), pericarditis (inflammation of the sac around the heart), and even heart attacks. These risks are especially heightened in individuals with pre-existing heart conditions.
• Neurological complications: Neurological complications can arise from influenza, including encephalitis (inflammation of the brain), myositis (inflammation of the muscles), and rhabdomyolysis (muscle damage). These conditions can lead to severe health issues and require immediate medical attention.
• Sepsis: Sepsis is a life-threatening condition that can result from the body's extreme response to infection, including influenza. It can lead to multi-organ failure and is a medical emergency that requires prompt treatment.
• Dehydration: Dehydration is another concern associated with influenza, especially in individuals experiencing vomiting and diarrhoea. This can lead to further complications if not managed appropriately.
• Exacerbation of chronic conditions: Influenza can exacerbate existing chronic conditions such as asthma, heart disease, diabetes, and kidney or liver disease. Individuals with these conditions may experience more severe symptoms and complications due to the flu.

Preventing influenza is crucial for maintaining good health, and several effective strategies are available to reduce the risk of infection. These include:

• Vaccination: The annual flu vaccine significantly reduces the risk of contracting influenza and its associated complications. Individuals need to receive the vaccine annually, as the virus can mutate and the vaccine is updated to provide the most effective protection.
• Good hygiene practices: This includes frequent handwashing with soap and water or using alcohol-based hand sanitisers. Covering coughs and sneezes with a tissue or elbow helps prevent respiratory droplets from spreading the virus. Avoiding close contact with sick individuals is also important to minimise exposure.
• Cleaning and disinfection: Regularly cleaning and disinfecting frequently touched surfaces, such as doorknobs, keyboards, and phones, can help reduce the risk of infection. These measures are particularly important in shared spaces, where the virus can easily spread.
• Healthy lifestyle: Maintaining a healthy lifestyle can boost the immune system and reduce susceptibility to influenza. This includes getting adequate sleep, eating a balanced diet, staying physically active, and managing stress.
• Antiviral medications: In some cases, antiviral medications may be prescribed as a preventive measure for individuals at high risk of complications or those who have been exposed to the virus. These medications can help reduce the severity and duration of influenza if taken early.

Diagnosing influenza typically involves a combination of clinical evaluation and laboratory testing. The diagnostic tests include:

• Symptom evaluation: Key indicators of influenza include a sudden onset of fever, cough, and other respiratory symptoms, such as a sore throat and nasal congestion. Muscle or body aches, headaches, and fatigue are also common.
• Rapid influenza diagnostic tests (RIDTs): Rapid influenza diagnostic tests (RIDTs) can detect influenza viral antigens in respiratory specimens, providing results within 10-15 minutes. However, these tests can have variable sensitivity and specificity, meaning they may not always accurately detect the virus.
• RT-PCR and viral culture: More accurate and sensitive tests, such as reverse transcription-polymerase chain reaction (RT-PCR) and viral culture, are considered the gold standard for influenza diagnosis. These tests detect the presence of the influenza virus in respiratory samples and are crucial for confirming a diagnosis.
• Clinical diagnosis: In certain cases, doctors may diagnose influenza based solely on symptoms, especially during peak flu season when the virus is widespread. This is particularly true for individuals exhibiting classic influenza symptoms, where laboratory testing may not be immediately necessary.

Managing influenza effectively involves a combination of antiviral medications, supportive care, and preventive measures. The treatment options include:

• Antiviral medications: Antiviral medications can be prescribed to treat influenza and are most effective when started within 48 hours of symptom onset. These medications can help reduce the duration of symptoms and lower the risk of complications, especially in high-risk individuals.
• Supportive care: This includes rest, staying hydrated, and using over-the-counter medications to relieve symptoms such as fever, aches, and congestion. Pain relievers can help alleviate fever and body aches.
• Monitoring for complications: Patients, particularly those at high risk for complications, should be closely monitored for worsening symptoms, such as pneumonia, difficulty breathing, chest pain, or severe dehydration. If these symptoms arise, seek medical attention immediately.
• Preventive measures: The annual flu vaccine is the most effective way to reduce the risk of contracting the virus. Additionally, practicing good hygiene, such as frequent handwashing and avoiding close contact with infected individuals, can help prevent the spread of influenza.
• Special considerations for high-risk groups: Individuals in high-risk groups, such as young children, elderly adults, pregnant women, and those with chronic medical conditions, may require more intensive management and should seek medical advice early in the illness.

Medications for Influenza

1. Neuraminidase Inhibitors (NAIs)

• Oseltamivir, Zanamivir, Peramivir: These antiviral agents inhibit the influenza neuraminidase enzyme, preventing the virus from being released from infected cells and limiting its spread within the body.

2. Cap-Dependent Endonuclease Inhibitor

This newer class of antiviral works by inhibiting viral RNA transcription, halting replication at an early stage:

• Baloxavir marboxil: Inhibits viral RNA polymerase, stopping the influenza virus from replicating early in its life cycle.

3. Supportive Medications

These do not target the virus but help relieve flu symptoms and manage complications:

• Antipyretics and Analgesics:
  • Paracetamol (acetaminophen) and ibuprofen for fever, headache, and muscle aches
  • Avoid aspirin in children and teens due to the risk of Reye’s syndrome
• Antitussives:
  • Dextromethorphan for dry cough
• Expectorants:
  • Guaifenesin to loosen mucus and improve cough effectiveness
• Decongestants:
  • Pseudoephedrine or phenylephrine to relieve nasal congestion
  • Use with caution in patients with hypertension
• Antihistamines:
  • Chlorpheniramine, diphenhydramine, or loratadine for a runny nose and sneezing

4. Preventive Strategies

• Annual Influenza Vaccination:
  • Most effective preventive measure
  • Available as inactivated (IIV), live-attenuated (LAIV), or recombinant (RIV) formulations
• Post-exposure Prophylaxis:
  • Oseltamivir, zanamivir, or baloxavir may be used for high-risk close contacts under medical supervision

If you belong to a high-risk group—such as young children, adults 65 years and older, pregnant individuals, or those with medical conditions like asthma, diabetes, or heart disease—it’s essential to contact your doctor if you suspect you have influenza. Additionally, seek medical care promptly if you experience severe symptoms, including difficulty breathing, chest pain, confusion, persistent vomiting, or if your symptoms improve but then return with fever and a worsening cough.

• Influenza is a contagious respiratory illness caused by influenza A or B viruses, which can lead to mild to severe symptoms and potentially life-threatening complications.
• Antiviral medications such as oseltamivir, zanamivir, peramivir, and baloxavir are crucial for managing influenza, particularly in high-risk patients or those with severe symptoms.
• Supportive care, including rest, hydration, and symptom management, is essential for all patients with influenza.
• Seek medical attention if you are at high risk for complications or experience severe symptoms, as early diagnosis and treatment can improve outcomes and prevent complications.
• Prevention measures such as annual vaccination, good hygiene practices, and avoiding close contact with infected individuals can help reduce the risk of contracting influenza.