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

What Is Influenza and How Is It Different from a Cold?

Influenza (flu) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. Unlike the common cold, which develops gradually with mild symptoms, the flu comes on suddenly with high fever (39-40C), severe body aches, extreme fatigue, and dry cough.

Influenza is one of the most common infectious diseases worldwide, affecting an estimated 5-15% of the global population each year. The illness is caused by influenza viruses, primarily types A and B, which circulate during seasonal epidemics. While often called "the flu," influenza should not be confused with stomach flu (gastroenteritis), which is caused by different viruses and primarily affects the digestive system rather than the respiratory tract.

The distinction between influenza and the common cold is important because the flu can lead to serious complications and requires different management. Cold symptoms typically include runny nose, sneezing, and mild fatigue, developing gradually over several days. In contrast, flu symptoms strike suddenly and are significantly more severe, often confining people to bed for several days. The systemic symptoms of flu, including muscle aches, headache, and extreme exhaustion, are rarely seen with common colds.

Seasonal influenza occurs most commonly during autumn and winter months in temperate climates, hence the term "flu season." In the Northern Hemisphere, this typically spans from October through March, while in the Southern Hemisphere, the season runs from April through September. The virus spreads easily in colder months when people spend more time indoors in close contact with others, and when lower humidity allows the virus to survive longer in the air.

Why does influenza occur seasonally?

Several factors contribute to the seasonal pattern of influenza outbreaks. Cold, dry air allows influenza virus particles to remain suspended in the air longer and travel farther when an infected person coughs or sneezes. Indoor heating systems reduce humidity further, creating ideal conditions for viral transmission. Additionally, people tend to congregate indoors during colder months, increasing opportunities for person-to-person spread.

The virus itself also plays a role in seasonality. Influenza viruses undergo continuous genetic changes through a process called antigenic drift, which allows them to evade immunity developed from previous infections or vaccinations. This is why new flu vaccines are developed each year to match the circulating strains, and why getting vaccinated annually is so important for protection.

What Are the Symptoms of Influenza?

The hallmark symptoms of influenza include sudden high fever (39-40C), severe body aches, dry cough, headache, extreme fatigue, sore throat, and chills. Symptoms typically appear 1-3 days after exposure and are most severe during the first 3-4 days. Unlike a cold, flu causes intense systemic symptoms that affect the entire body.

Influenza symptoms are notable for their sudden onset and severity. Most people with the flu describe feeling fine one moment and then, within hours, experiencing overwhelming fatigue, fever, and body aches. This rapid progression is a key distinguishing feature that helps differentiate flu from colds and other respiratory infections that develop more gradually.

The fever associated with influenza is typically high, ranging from 39 to 40 degrees Celsius (102 to 104 degrees Fahrenheit), and may persist for up to a week. However, fever patterns can vary, and some people, particularly older adults, may have flu without fever or with only low-grade temperature elevation. This is important to recognize because lack of fever does not rule out influenza, especially in high-risk populations.

Body aches and muscle pain (myalgia) are among the most distressing symptoms of influenza. Unlike the mild discomfort sometimes experienced with colds, flu-related body aches are often severe and widespread, affecting muscles throughout the body. Many people describe feeling as though they have been "hit by a truck." These aches result from the immune system's inflammatory response to the virus and can make even simple movements uncomfortable.

Common symptoms in adults

• High fever: Typically 39-40C (102-104F), lasting 3-7 days
• Dry, persistent cough: May last 2-3 weeks even after other symptoms resolve
• Severe body aches: Affecting muscles and joints throughout the body
• Headache: Often severe, located behind the eyes or across the forehead
• Extreme fatigue: May persist for 2-3 weeks after illness
• Chills and sweating: Alternating as fever rises and falls
• Sore throat: Usually mild to moderate
• Runny or stuffy nose: Less prominent than with colds

Symptoms in children

Children with influenza often present differently than adults. In addition to the typical symptoms, young children may experience gastrointestinal symptoms such as nausea, vomiting, diarrhea, and stomach pain that are less common in adults. Children under two years of age are at particularly high risk for complications and may develop more severe illness.

Infants may show signs of flu through irritability, poor feeding, lethargy, and reduced activity levels. They may not be able to communicate their symptoms, making it important for caregivers to monitor for fever, breathing difficulties, and changes in behavior. Any infant under three months with fever should be evaluated by a healthcare provider promptly.

How long do symptoms last?

Most people with uncomplicated influenza begin to feel better within 3-7 days, though complete recovery typically takes one to two weeks. Fever usually resolves within a week, but cough and fatigue can persist for two to three weeks or longer. Returning to normal activities too quickly can prolong recovery and increase the risk of complications.

The timeline of symptom resolution varies by individual and depends on factors including age, overall health, and immune status. Healthy adults generally recover faster than older adults or those with chronic conditions. Children may take longer to recover completely and should be kept home from school or childcare until they have been fever-free for at least 24 hours without fever-reducing medication.

How Does Influenza Spread?

Influenza spreads primarily through respiratory droplets produced when infected people cough, sneeze, talk, or even breathe. You can also become infected by touching contaminated surfaces and then touching your face. The virus can spread up to 6 feet from an infected person, and you are contagious from 1 day before symptoms start until about 5-7 days after becoming sick.

Understanding how influenza spreads is essential for preventing infection and protecting others. The primary mode of transmission is through respiratory droplets that are expelled when an infected person coughs, sneezes, laughs, or speaks. These droplets can travel through the air and land on the mouths, noses, or eyes of people nearby, or be inhaled directly into the lungs. This is why maintaining distance from sick individuals and wearing masks in crowded spaces during flu season can help reduce transmission.

The virus can also spread through contact transmission. Influenza viruses can survive on hard surfaces such as doorknobs, light switches, and phones for up to 24 hours, and on soft surfaces like tissues for about 15 minutes. If you touch a contaminated surface and then touch your eyes, nose, or mouth, you can introduce the virus into your body. This highlights the importance of frequent handwashing and avoiding touching your face, especially during flu season.

Research has also shown that influenza can spread through fine aerosol particles that remain suspended in the air for longer periods, particularly in poorly ventilated indoor spaces. This airborne transmission may play a more significant role than previously thought, especially in crowded indoor environments. Improving ventilation in indoor spaces can help reduce this mode of transmission.

When are you most contagious?

One of the most challenging aspects of influenza prevention is that infected individuals can spread the virus before they even know they are sick. You can begin transmitting the virus to others about one day before your symptoms start, when you feel completely healthy. This pre-symptomatic transmission makes it difficult to prevent spread through isolation alone and underscores the importance of vaccination.

Contagiousness peaks during the first three to four days of illness, when symptoms are most severe and viral shedding is highest. Most healthy adults remain contagious for about 5-7 days after becoming sick. However, young children and people with weakened immune systems may shed virus for longer periods, potentially up to two weeks or more, extending the time during which they can infect others.

Who Is at Higher Risk for Complications?

While anyone can get seriously ill from influenza, certain groups face higher risk of severe complications: adults over 65, children under 5 (especially under 2), pregnant women, people with chronic conditions like heart disease, lung disease, or diabetes, and those with weakened immune systems. These high-risk groups are prioritized for annual vaccination.

Although most healthy people recover from influenza without complications, the virus can cause severe illness requiring hospitalization and can even be fatal. Globally, seasonal influenza causes an estimated 290,000 to 650,000 respiratory deaths each year, with the vast majority occurring in high-risk populations. Understanding who is at increased risk helps target prevention efforts and ensures that those most vulnerable receive appropriate care.

Age is one of the most significant risk factors for influenza complications. Adults aged 65 and older account for the majority of flu-related hospitalizations and deaths. This increased vulnerability results from age-related changes in immune function (immunosenescence) and a higher prevalence of underlying chronic conditions. Similarly, children under 5, and especially those under 2 years of age, have immature immune systems that make them more susceptible to severe flu illness.

Pregnancy alters the immune system and increases cardiovascular and respiratory demands, making pregnant women more vulnerable to severe influenza. Flu during pregnancy can also harm the developing baby, increasing the risk of premature birth and other complications. The good news is that flu vaccination during pregnancy is safe and effective, protecting both the mother and the newborn during the first few months of life.

Chronic conditions that increase risk

• Chronic lung disease: Including asthma, COPD, cystic fibrosis, and pulmonary fibrosis
• Heart disease: Including coronary artery disease, congestive heart failure, and congenital heart defects
• Diabetes: Both type 1 and type 2 diabetes increase infection risk
• Kidney disease: Chronic kidney disease and those on dialysis
• Liver disease: Including cirrhosis and chronic hepatitis
• Neurological conditions: Including stroke, epilepsy, muscular dystrophy, and cerebral palsy
• Immunocompromising conditions: HIV/AIDS, cancer treatment, organ transplant recipients, and those on immunosuppressive medications
• Severe obesity: Body mass index (BMI) of 40 or higher

People with these conditions should prioritize annual flu vaccination, seek medical care early if they develop flu symptoms, and may be candidates for antiviral treatment even if their symptoms are mild. Healthcare providers may recommend starting antiviral medications promptly in high-risk individuals rather than waiting to see how the illness progresses.

When Should You Seek Medical Care?

Seek medical care if you have high fever lasting more than 4 days, fever that returns after improving, difficulty breathing even at rest, severe chest pain, confusion, or sudden worsening of symptoms. People in high-risk groups should contact a healthcare provider early. Call emergency services immediately for severe breathing difficulty, persistent chest pain, confusion, or inability to keep fluids down.

Most people with influenza recover at home with rest, fluids, and over-the-counter medications for symptom relief. However, it is important to recognize warning signs that indicate the need for medical evaluation. Early medical attention can be particularly important for people in high-risk groups, where prompt treatment with antiviral medications can prevent complications.

Contact your healthcare provider or visit an urgent care clinic if you have influenza symptoms and belong to a high-risk group, even if your symptoms are mild. Early treatment with antiviral medications is most effective when started within 48 hours of symptom onset, so prompt evaluation is important. Your healthcare provider can also advise you on warning signs specific to your underlying conditions.

For otherwise healthy individuals, medical care may be needed if symptoms are unusually severe or prolonged. Fever that persists beyond 4 days, or fever that initially improves and then returns, may indicate a secondary bacterial infection such as pneumonia that requires antibiotics. Progressive worsening of symptoms rather than gradual improvement is also a reason to seek evaluation.

What Can You Do at Home to Feel Better?

Rest is essential for recovery from influenza. Stay home to avoid spreading the virus, drink plenty of fluids to prevent dehydration, use paracetamol or ibuprofen for fever and pain, keep the room cool, and elevate your head while sleeping to ease breathing. Most symptoms improve within 3-7 days, though fatigue may persist for 2-3 weeks.

While there is no cure for influenza, self-care measures can significantly improve comfort and support recovery. The body needs energy to fight the infection, making rest one of the most important aspects of flu recovery. This means staying home from work or school not just to prevent spreading the virus to others, but to allow your body to heal. Pushing through illness often prolongs recovery and increases the risk of complications.

Adequate hydration is crucial during any illness with fever. Fever increases fluid loss through sweating and rapid breathing, while poor appetite often reduces fluid intake. Dehydration can worsen symptoms and delay recovery. Aim to drink water, clear broths, herbal teas, or electrolyte solutions throughout the day. Avoid alcohol and limit caffeine, which can contribute to dehydration. If you have difficulty keeping fluids down, take small sips frequently rather than drinking large amounts at once.

Try to eat when you can, even if you do not feel hungry. Your body needs nutrition to power the immune response. Easily digestible foods like soup, toast, crackers, and bland foods are often easier to tolerate. Some people find that warm liquids like chicken soup provide comfort and help with congestion, and there is some evidence that soup has mild anti-inflammatory properties.

Managing fever and pain

Over-the-counter medications can provide significant relief from flu symptoms. Paracetamol (acetaminophen) and ibuprofen are both effective for reducing fever and relieving body aches, headache, and sore throat. Follow the package directions carefully and do not exceed recommended doses. If you are uncertain which medication is appropriate for you, consult a pharmacist or healthcare provider.

For children, paracetamol can be given from 3 months of age and ibuprofen from 6 months of age, both available in formulations appropriate for children. Follow dosing instructions based on the child's weight, and never give aspirin to children or teenagers with flu due to the risk of Reye's syndrome, a rare but serious condition.

Relieving congestion and cough

Nasal congestion can be relieved with saline nasal drops or spray, which are safe for all ages. Decongestant nasal sprays can provide temporary relief but should not be used for more than 10 consecutive days to avoid rebound congestion. Oral decongestants are available but may not be suitable for everyone, particularly those with high blood pressure, heart disease, or certain other conditions.

Cough is often one of the most persistent flu symptoms. Drinking warm liquids, using throat lozenges, and honey (for those over 1 year old) can help soothe a sore throat and suppress cough. Over-the-counter cough medications have limited evidence for effectiveness, but some people find them helpful. Avoid smoking and exposure to secondhand smoke, which irritates the airways and worsens cough.

How Is Influenza Treated?

Most people with influenza recover with rest and supportive care. Antiviral medications like oseltamivir (Tamiflu) or zanamivir (Relenza) can shorten illness duration by 1-2 days and reduce severity if started within 48 hours of symptom onset. These prescription medications are recommended primarily for high-risk individuals and those with severe illness. Antibiotics do not work against influenza since it is caused by a virus.

The mainstay of influenza treatment for most healthy people is supportive care: rest, hydration, and symptom management with over-the-counter medications. The immune system will eventually clear the virus, and treatment focuses on helping you feel better while your body does this work. Most uncomplicated flu resolves within one to two weeks without specific antiviral treatment.

However, prescription antiviral medications are available and can be valuable in certain situations. These medications work by interfering with the influenza virus's ability to replicate in your body. When started within 48 hours of symptom onset, antivirals can reduce the duration of illness by approximately one to two days and decrease the severity of symptoms. Perhaps more importantly, they can reduce the risk of serious complications like pneumonia.

Antiviral treatment is most commonly recommended for people at high risk of complications, including those aged 65 and older, children under 5 years, pregnant women, and people with chronic medical conditions. Treatment may also be recommended for anyone with severe illness requiring hospitalization, regardless of their risk status. In these situations, antiviral treatment can be lifesaving.

Available antiviral medications

• Oseltamivir (Tamiflu): Available as capsules or liquid suspension, taken twice daily for 5 days. Can be used in adults and children of any age.
• Zanamivir (Relenza): Inhaled powder using a special device, taken twice daily for 5 days. Approved for those 7 years and older; not recommended for people with respiratory conditions like asthma or COPD.
• Peramivir (Rapivab): Given as a single intravenous dose in healthcare settings.
• Baloxavir (Xofluza): Single oral dose, approved for those 12 years and older.

Hospital treatment

Some people with severe influenza require hospitalization for more intensive care. This may include intravenous fluids for dehydration, supplemental oxygen for breathing difficulties, and close monitoring for complications. In severe cases, mechanical ventilation may be needed for respiratory failure. Hospital treatment also allows for intravenous administration of antiviral medications and treatment of any secondary infections.

How Can You Prevent Influenza?

Annual flu vaccination is the single most effective way to prevent influenza and is recommended for everyone 6 months and older. Additional prevention measures include frequent handwashing, avoiding close contact with sick people, covering coughs and sneezes, and staying home when ill. Good ventilation in indoor spaces also helps reduce transmission.

Prevention is always better than treatment, and this is especially true for influenza. A combination of vaccination and everyday preventive actions can significantly reduce your risk of getting the flu and help protect those around you. While no prevention strategy is 100% effective, these measures working together provide substantial protection.

The cornerstone of flu prevention is annual vaccination. Flu vaccines are developed each year to match the influenza strains expected to circulate during the upcoming season. Getting vaccinated not only protects you but also contributes to community immunity, reducing transmission and protecting those who cannot be vaccinated or who have weaker immune responses to the vaccine.

Who should get the flu vaccine?

The World Health Organization and public health authorities recommend annual flu vaccination for everyone 6 months of age and older. However, vaccination is particularly important for high-risk groups and those who may transmit the virus to vulnerable individuals:

• Adults 65 years and older
• Children aged 6 months to 5 years
• Pregnant women (vaccination is safe during any trimester)
• People with chronic medical conditions
• Healthcare workers and caregivers
• Residents of nursing homes and long-term care facilities
• People living with or caring for high-risk individuals

The flu vaccine is typically available from early autumn each year. It takes about two weeks after vaccination for protective antibodies to develop, so getting vaccinated early in the season provides the best protection. However, vaccination later in the season is still beneficial as flu activity can continue into spring.

Everyday prevention measures

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

• Wash your hands frequently: Use soap and water for at least 20 seconds, especially after coughing, sneezing, or being in public places. Use alcohol-based hand sanitizer when soap is not available.
• Avoid touching your face: The virus enters through the eyes, nose, and mouth. Keeping hands away from your face reduces transmission risk.
• Cover coughs and sneezes: Use a tissue or your elbow, not your hands. Dispose of tissues immediately and wash hands afterward.
• Keep distance from sick people: Avoid close contact with anyone showing flu symptoms when possible.
• Clean frequently touched surfaces: Doorknobs, light switches, phones, and keyboards can harbor viruses.
• Stay home when sick: This protects others and allows you to recover properly.
• Improve indoor ventilation: Open windows when possible and use air filtration in enclosed spaces.

What Complications Can Influenza Cause?

Influenza can lead to serious complications including pneumonia (viral or bacterial), sinusitis, ear infections (especially in children), and worsening of chronic conditions. Less common but serious complications include myocarditis (heart inflammation), encephalitis (brain inflammation), and multi-organ failure. Complications are more likely in high-risk groups.

While most people with influenza recover completely, the virus can sometimes cause serious complications that require medical treatment. Understanding potential complications helps recognize warning signs early and underscores the importance of prevention, especially for vulnerable populations.

Pneumonia is the most common serious complication of influenza. The flu virus can directly cause viral pneumonia, or it can damage the respiratory tract in ways that allow bacteria to cause secondary bacterial pneumonia. Pneumonia symptoms include high fever, productive cough (with yellow, green, or blood-tinged sputum), chest pain with breathing, and shortness of breath. Bacterial pneumonia following flu can be severe and sometimes fatal, particularly in older adults.

Sinus infections (sinusitis) and ear infections (otitis media) are common complications, especially in children. These occur when inflammation and mucus buildup from the flu create conditions favorable for bacterial growth. Symptoms may include facial pain or pressure, ear pain, and persistent symptoms beyond the typical flu duration. These secondary infections often require antibiotic treatment.

Serious complications

• Pneumonia: Either viral pneumonia from the flu itself or bacterial pneumonia as a secondary infection
• Myocarditis: Inflammation of the heart muscle, which can cause chest pain, rapid heartbeat, and heart failure
• Encephalitis: Inflammation of the brain, causing confusion, seizures, or altered consciousness
• Rhabdomyolysis: Breakdown of muscle tissue, sometimes seen with severe muscle pain in children
• Multi-organ failure: In severe cases, flu can trigger systemic inflammation affecting multiple organs
• Worsening of chronic conditions: Flu can exacerbate asthma, COPD, heart failure, and other underlying diseases

Children sometimes develop an unusual complication characterized by severe leg pain, particularly in the calves, following influenza infection. This condition, called benign acute childhood myositis, typically occurs as other flu symptoms are resolving and causes intense pain that may make walking difficult. While frightening, it is usually harmless and resolves within a few days.

Influenza During Pregnancy and Breastfeeding

Pregnant women are at higher risk for serious influenza complications and should be prioritized for annual flu vaccination, which is safe during any trimester. If you develop flu during pregnancy, contact your healthcare provider early as antiviral treatment may be recommended. Breastfeeding can continue safely if you have the flu, and breast milk provides antibodies that help protect your baby.

Pregnancy causes changes in the immune system, heart, and lungs that increase susceptibility to severe influenza illness. Pregnant women with flu are more likely to be hospitalized and are at higher risk of complications than non-pregnant women of the same age. Influenza during pregnancy can also affect the developing baby, increasing the risk of preterm birth, low birth weight, and stillbirth.

The good news is that flu vaccination during pregnancy is safe and provides important protection. The vaccine cannot cause flu because it does not contain live virus. Getting vaccinated during pregnancy protects both the mother and the baby. Antibodies produced by the mother cross the placenta and provide the newborn with protection during the first few months of life, before the baby is old enough to be vaccinated.

If you are pregnant and develop flu symptoms, contact your healthcare provider promptly. Antiviral medications are recommended for pregnant women with suspected or confirmed influenza, as the benefits of treatment outweigh potential risks. Early treatment can reduce the severity of illness, shorten the duration of symptoms, and prevent serious complications.