Influenza (Flu): Symptoms, Treatment & When to Seek Care

What Is Influenza (Flu)?

Influenza, commonly known as the flu, is a highly contagious respiratory infection caused by influenza viruses. It affects the nose, throat, and lungs, causing symptoms ranging from mild to severe. The flu is most common during winter months, which is why it's often called seasonal influenza.

Influenza is a significant global health concern, affecting an estimated 5-15% of the world's population each year. While most people recover without complications, influenza can cause severe illness and even death, particularly among vulnerable populations. The World Health Organization estimates that seasonal influenza causes 3-5 million cases of severe illness and 290,000-650,000 respiratory deaths annually worldwide.

The influenza virus belongs to the Orthomyxoviridae family and is classified into four types: A, B, C, and D. Types A and B cause the seasonal flu epidemics that affect humans each winter. Influenza A viruses are further categorized by their surface proteins: hemagglutinin (H) and neuraminidase (N), leading to subtypes like H1N1 and H3N2. These viruses constantly evolve through antigenic drift and shift, which explains why new vaccines are needed each year and why previous infection doesn't provide lasting immunity.

The seasonal nature of influenza in temperate climates is well-documented, with epidemics typically occurring between October and May in the Northern Hemisphere and April to September in the Southern Hemisphere. This seasonality is influenced by factors including temperature, humidity, and human behavior patterns such as spending more time indoors during colder months, which facilitates virus transmission.

How Influenza Differs from Other Respiratory Infections

Understanding the distinction between influenza and other respiratory infections is crucial for appropriate management. While the common cold, COVID-19, and influenza share some symptoms, they differ significantly in severity, onset, and potential complications.

The flu typically causes more severe systemic symptoms compared to the common cold. Cold symptoms develop gradually over several days and primarily affect the nose and throat, whereas flu symptoms appear suddenly and affect the entire body. Fatigue from a cold is usually mild, but flu-related fatigue can be debilitating and last for weeks after other symptoms resolve.

What Are the Symptoms of Influenza?

Influenza symptoms typically appear suddenly and include high fever (39-40°C/102-104°F), severe body aches, headache, extreme fatigue, dry cough, sore throat, and nasal congestion. Children may also experience nausea, vomiting, and diarrhea. Symptoms are usually most severe during the first 3-4 days.

The hallmark of influenza is its sudden onset. Many people can pinpoint the exact hour when they started feeling ill, unlike the gradual progression of cold symptoms. This rapid onset is a key distinguishing feature that helps differentiate flu from other respiratory infections. The virus typically incubates for 1-3 days before symptoms appear, during which time an infected person can unknowingly spread the virus to others.

Fever is one of the most prominent symptoms of influenza, typically ranging from 39 to 40°C (102-104°F). The fever usually lasts 3-7 days and is often accompanied by chills and sweating. In adults, fever tends to be more pronounced during the first few days and gradually decreases, while children may experience higher fevers that last longer. The body uses fever as a defense mechanism against viral infections, as influenza viruses replicate less efficiently at higher temperatures.

Respiratory symptoms are central to the flu experience. A dry, persistent cough is common and can last for 1-2 weeks after other symptoms have resolved. Unlike the productive cough often seen with bacterial infections, the flu cough is typically dry and hacking, resulting from inflammation of the airways. Sore throat and nasal congestion also occur, though they are generally less prominent than in the common cold. People with pre-existing respiratory conditions like asthma or COPD may experience significant worsening of their symptoms during influenza infection.

The systemic symptoms of influenza – body aches, headache, and fatigue – distinguish it from milder respiratory infections. Muscle aches (myalgia) can be severe, affecting the back, arms, and legs. Headache is often described as throbbing and located behind the eyes. The fatigue associated with influenza can be profound and debilitating, often persisting for 2-3 weeks after other symptoms have resolved. This prolonged fatigue is one reason why returning to normal activities too quickly after flu can delay full recovery.

Symptoms in Children

Children may present with symptoms that differ from adults. In addition to the typical respiratory symptoms, children under age 2 are more likely to experience gastrointestinal symptoms including nausea, vomiting, and diarrhea. Young children may also develop ear infections (otitis media) as a complication of influenza. Infants may show non-specific symptoms such as irritability, poor feeding, and lethargy, making diagnosis more challenging.

It's important to monitor children closely during flu illness, as they can deteriorate rapidly. Warning signs that require immediate medical attention include rapid or difficult breathing, bluish skin color, severe or persistent vomiting, not drinking adequate fluids, extreme irritability or unusual sleepiness, and fever with a rash.

How Does the Flu Spread?

Influenza spreads primarily through respiratory droplets when infected people cough, sneeze, talk, or breathe. It can also spread by touching contaminated surfaces and then touching the face. You can transmit the virus 1 day before symptoms appear and remain contagious for 5-7 days after becoming ill.

Understanding how influenza spreads is essential for effective prevention. The virus primarily travels through respiratory droplets expelled when an infected person coughs, sneezes, laughs, or even talks. These droplets can travel up to 2 meters (6 feet) and land in the mouths or noses of nearby people, or be inhaled into the lungs. This is why maintaining distance from sick individuals is an important preventive measure.

In addition to droplet transmission, influenza can spread through contact with contaminated surfaces. The virus can survive on hard surfaces for up to 48 hours and on hands for about 5 minutes. When a person touches a contaminated surface and then touches their eyes, nose, or mouth, they can become infected. This indirect transmission route highlights the importance of frequent handwashing and avoiding touching the face with unwashed hands.

Recent research has also demonstrated that influenza can spread through aerosols – tiny particles that remain suspended in the air for longer periods and can travel further distances. This airborne transmission is particularly relevant in poorly ventilated indoor spaces where many people gather. Improving ventilation and air filtration can help reduce this transmission route.

When Are You Most Contagious?

One of the challenging aspects of influenza control is that infected individuals can spread the virus before they know they're sick. The contagious period typically begins 1 day before symptoms appear and continues for 5-7 days after illness onset. You are most contagious during the first 3-4 days when symptoms are most severe and viral shedding is highest.

Certain populations may be contagious for longer periods. Children and people with weakened immune systems can shed the virus for 10 days or more. This extended contagious period is one reason why schools and healthcare facilities can be significant sites of influenza transmission.

Who Is at Higher Risk of Severe Illness?

High-risk groups for severe influenza complications include adults over 65, children under 5 (especially under 2), pregnant women, people with chronic conditions (heart disease, lung disease, diabetes, weakened immune system), nursing home residents, and healthcare workers.

While anyone can get the flu, certain groups face significantly higher risks of developing severe illness, complications, and death. Understanding these risk factors helps guide prevention strategies and decisions about when to seek medical care.

Age is one of the most important risk factors. Adults 65 years and older account for the majority of influenza-related hospitalizations and deaths. This increased vulnerability is due to age-related changes in immune function, higher prevalence of chronic medical conditions, and reduced response to vaccination. Similarly, children under 5 years old, particularly those under 2, are at increased risk due to their immature immune systems.

Pregnancy increases the risk of severe influenza illness due to changes in the immune system, heart, and lungs that occur during pregnancy. Pregnant women with influenza are more likely to be hospitalized than non-pregnant women of the same age. Influenza infection during pregnancy can also harm the fetus, potentially causing preterm birth or low birth weight. Importantly, the flu vaccine is safe during pregnancy and protects both mother and newborn.

Chronic medical conditions significantly increase influenza risk. These include:

• Respiratory conditions: Asthma, COPD, and cystic fibrosis can worsen dramatically with influenza infection
• Cardiovascular disease: Heart failure and coronary artery disease increase complication risk
• Metabolic disorders: Diabetes affects immune function and healing
• Kidney and liver disease: These conditions impair the body's ability to fight infection
• Immunocompromising conditions: HIV/AIDS, cancer treatment, and immunosuppressive medications reduce the body's defenses
• Neurological conditions: Conditions affecting breathing or swallowing increase pneumonia risk
• Obesity: BMI of 40 or higher is associated with increased flu complications

When Should You Seek Medical Care?

Most people recover from flu at home without medical care. Seek medical attention if: high fever persists beyond 4 days, fever improves then returns, breathing becomes difficult, you suddenly feel much worse, or you're in a high-risk group. Call emergency services immediately for severe breathing difficulty, chest pain, confusion, or bluish skin color.

The majority of people with influenza experience a self-limiting illness and recover within 1-2 weeks with supportive home care. However, knowing when medical attention is needed can prevent serious complications and save lives. The decision to seek care depends on symptom severity, duration, and individual risk factors.

Several symptoms warrant prompt medical evaluation. Persistent high fever that doesn't improve after 4 days of illness suggests either a complicated infection or another condition. A fever that initially improves and then returns, especially with worsening respiratory symptoms, may indicate a secondary bacterial infection such as pneumonia. Difficulty breathing, even when resting, is always a concerning symptom that requires immediate evaluation.

People in high-risk groups should contact a healthcare provider early in their illness, even if symptoms seem mild. This is because antiviral medications are most effective when started within 48 hours of symptom onset, and high-risk individuals may benefit from early treatment even if initially presenting with mild symptoms. If you have chronic medical conditions, are pregnant, over 65, or immunocompromised, don't wait to see if symptoms worsen before seeking advice.

Emergency Signs in Children

Children can deteriorate rapidly with influenza. Parents should seek emergency care if their child shows fast or labored breathing, bluish skin color, not drinking enough fluids, severe irritability (doesn't want to be held), fever with a rash, or flu symptoms that improve then return with fever and worse cough.

How Is Influenza Treated?

Most flu cases are treated with rest, fluids, and symptom relief using over-the-counter medications like paracetamol or ibuprofen for fever and pain. Antiviral medications (oseltamivir/Tamiflu, zanamivir/Relenza) may be prescribed for high-risk patients or severe illness, ideally within 48 hours of symptom onset. Antibiotics do not work against influenza.

Treatment for influenza focuses on managing symptoms and preventing complications. For most healthy individuals, supportive care at home is sufficient, while antiviral medications may be appropriate for high-risk groups or severe cases. Understanding the available treatment options helps patients make informed decisions and set realistic expectations for recovery.

Home Care and Self-Treatment

The cornerstone of flu treatment is supportive care. Rest is essential – your body needs energy to fight the infection, and trying to push through illness can prolong recovery. Many people make the mistake of returning to work or school too soon, which can delay full recovery and spread the infection to others.

Adequate hydration is crucial during influenza illness. Fever increases fluid loss through sweating, and illness often reduces appetite and thirst. Aim to drink more fluids than usual, including water, clear broths, and electrolyte solutions. Signs of dehydration include dark urine, dry mouth, dizziness when standing, and reduced urination. If you're caring for a sick child, monitor their fluid intake closely and offer small amounts frequently.

Over-the-counter medications can help manage symptoms:

• Fever and pain relief: Paracetamol (acetaminophen) or ibuprofen can reduce fever and relieve body aches, headache, and sore throat. Follow dosing instructions carefully and avoid exceeding maximum daily doses
• Nasal congestion: Saline nasal drops or spray can help relieve congestion. Decongestant nasal sprays should not be used for more than 10 consecutive days
• Cough: Over-the-counter cough medications may provide some relief, though their effectiveness is limited. Honey can soothe coughs in adults and children over 1 year old
• Sore throat: Throat lozenges, warm liquids, and gargling with salt water can provide relief

Antiviral Medications

Antiviral medications can reduce the duration of flu illness by 1-2 days and may reduce the risk of complications. They work by preventing the virus from replicating in the body. The most commonly used antivirals for influenza are oseltamivir (Tamiflu), zanamivir (Relenza), and in some regions, baloxavir (Xofluza).

Antivirals are most effective when started within 48 hours of symptom onset, though they may still provide benefit when started later in high-risk patients or those with severe illness. They are typically recommended for:

• People hospitalized with influenza
• Those with severe or progressive illness
• People at high risk for complications (elderly, pregnant, chronic conditions)
• Some healthcare workers and household contacts of high-risk individuals

A doctor must prescribe antiviral medications after evaluating your specific situation. These medications can have side effects, and the decision to use them balances potential benefits against risks.

Why Antibiotics Don't Help

Antibiotics are designed to kill bacteria, not viruses. Since influenza is caused by a virus, antibiotics are completely ineffective against it. Taking antibiotics unnecessarily can cause side effects and contributes to antibiotic resistance – a serious global health threat. However, if you develop a secondary bacterial infection (such as bacterial pneumonia) as a complication of flu, antibiotics may be needed to treat that infection.

What Complications Can Influenza Cause?

Influenza complications include pneumonia (viral or bacterial), sinusitis, ear infections (especially in children), bronchitis, and worsening of chronic conditions. Rare but serious complications include myocarditis (heart inflammation), encephalitis (brain inflammation), and multi-organ failure. Complications are most common in high-risk groups.

While most people recover from influenza without problems, the virus can cause significant complications, particularly in vulnerable populations. Understanding potential complications helps patients recognize warning signs and seek appropriate care.

Pneumonia is the most common serious complication of influenza and a leading cause of flu-related hospitalizations and deaths. It can occur as primary viral pneumonia (caused directly by the flu virus) or secondary bacterial pneumonia (when bacteria invade flu-damaged lungs). Bacterial pneumonia typically develops a few days after flu symptoms improve, presenting as a return of fever with worsening cough, increased sputum production, and difficulty breathing.

Other respiratory complications include acute bronchitis (inflammation of the airways), sinusitis (sinus infection), and worsening of underlying respiratory conditions like asthma and COPD. Children are particularly susceptible to ear infections (otitis media) as a complication of influenza, and young children may develop croup, characterized by a barking cough and difficulty breathing.

Less common but serious complications include:

• Myocarditis and pericarditis: Inflammation of the heart muscle or surrounding tissue
• Encephalitis: Inflammation of the brain, causing confusion, seizures, or altered consciousness
• Guillain-Barré syndrome: A rare nerve disorder causing muscle weakness
• Rhabdomyolysis: Breakdown of muscle tissue, which can damage kidneys
• Multi-organ failure: In severe cases, particularly with secondary sepsis

An interesting note for parents: some children develop severe leg pain (myositis) during or after influenza infection. While this can be alarming, it is usually temporary and resolves within a few days. However, severe muscle pain with dark urine should prompt immediate medical evaluation.

How Can You Prevent Influenza?

The most effective prevention is annual influenza vaccination, which reduces illness risk by 40-60% and significantly reduces severe complications. Additional prevention measures include frequent handwashing, avoiding touching your face, maintaining distance from sick people, covering coughs and sneezes, and staying home when ill.

Preventing influenza involves a combination of vaccination and everyday preventive actions. While no prevention method is 100% effective, these strategies working together significantly reduce your risk of getting the flu and spreading it to others.

Influenza Vaccination

Annual vaccination is the single most effective way to prevent influenza. The vaccine is updated each year to match the circulating strains predicted by global surveillance systems. Vaccination is recommended for everyone 6 months of age and older, with particular emphasis on high-risk groups.

The flu vaccine's effectiveness varies from season to season, typically ranging from 40-60% when the vaccine is well-matched to circulating strains. Even when effectiveness is moderate, vaccination provides several important benefits:

• Reduces the risk of getting sick with the flu
• Reduces severity of illness in those who do get infected
• Significantly reduces the risk of hospitalization
• Reduces the risk of flu-related death
• Protects vulnerable people through community immunity

The best time to get vaccinated is before flu season begins, ideally in autumn (September-November in the Northern Hemisphere). However, vaccination later in the season still provides benefit. It takes about two weeks after vaccination for protective antibodies to develop.

Common misconceptions about the flu vaccine include the belief that it can cause the flu – it cannot. The injectable vaccine contains inactivated (killed) virus that cannot cause infection. Some people experience mild side effects like soreness at the injection site or low-grade fever, which are signs that the immune system is responding and are not the flu itself.

Everyday Prevention Measures

In addition to vaccination, simple hygiene practices can significantly reduce flu transmission:

• Wash hands frequently: Use soap and water for at least 20 seconds, especially after being in public, before eating, and after touching surfaces. When soap isn't available, use alcohol-based hand sanitizer
• Avoid touching your face: The flu virus enters through the eyes, nose, and mouth. Touching your face with contaminated hands is a common transmission route
• Cover coughs and sneezes: Use your elbow or a tissue, not your hands. Dispose of tissues immediately
• Maintain distance from sick people: When possible, stay at least 1-2 meters away from people showing flu symptoms
• Stay home when sick: If you develop flu symptoms, stay home until you've been fever-free for at least 24 hours without using fever-reducing medication
• Clean frequently touched surfaces: Doorknobs, light switches, phones, and keyboards can harbor viruses
• Consider outdoor activities: When flu is circulating widely, outdoor gatherings are safer than indoor ones due to better ventilation

Influenza During Pregnancy

Pregnancy increases the risk of severe influenza illness. Flu vaccination during pregnancy is safe and recommended – it protects both mother and newborn baby. Pregnant women who develop flu symptoms should contact their healthcare provider promptly as antiviral treatment may be recommended.

Pregnancy causes changes to the immune system, heart, and lungs that increase susceptibility to severe influenza illness. Pregnant women with flu are more likely to be hospitalized than non-pregnant women of the same age. Additionally, influenza during pregnancy has been associated with adverse outcomes including preterm birth, low birth weight, and in severe cases, stillbirth.

The flu vaccine is safe during any trimester of pregnancy and is strongly recommended. Vaccination during pregnancy provides dual protection: it protects the pregnant woman herself and provides passive immunity to the newborn baby through antibodies transferred during pregnancy and through breast milk. This protection is particularly valuable because infants under 6 months cannot receive the flu vaccine.

If you're pregnant and develop flu symptoms, contact your midwife or doctor promptly. While mild symptoms can often be managed at home, you may be advised to start antiviral medication to reduce the risk of complications. Continue your regular prenatal care, and watch for warning signs that require immediate medical attention.

Breastfeeding while you have the flu is safe and recommended. Breast milk provides antibodies and other factors that protect your baby against infection. Take precautions to avoid direct transmission by washing hands before handling the baby and considering wearing a mask while nursing if you have respiratory symptoms.