Influenza (Flu): Symptoms, Causes & Treatment Guide

What Is Influenza and What Causes It?

Influenza (flu) is a highly contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs. It spreads through respiratory droplets when infected people cough, sneeze, or talk. The flu affects 5-15% of the global population annually and is most common during winter months, earning it the name "seasonal flu."

Influenza is fundamentally different from a common cold, despite sharing some symptoms. The flu is caused by specific influenza viruses (types A, B, and rarely C), while colds are caused by various other viruses including rhinoviruses. This distinction matters because influenza can cause severe illness and even death, particularly in vulnerable populations, while colds are typically mild and self-limiting.

The influenza virus constantly evolves through a process called antigenic drift, which is why new vaccine formulations are needed each year. Occasionally, major changes occur (antigenic shift), which can lead to pandemic strains. This viral evolution explains why you can get the flu multiple times throughout your life and why immunity from a previous infection doesn't guarantee protection against future strains.

Seasonal influenza epidemics occur worldwide, typically during autumn and winter in temperate regions. In the Northern Hemisphere, flu season usually runs from October through May, with peak activity between December and February. The World Health Organization estimates that influenza causes 3-5 million cases of severe illness globally each year, with 290,000-650,000 respiratory deaths.

Types of Influenza Viruses

There are four types of influenza viruses: A, B, C, and D. Types A and B are responsible for seasonal flu epidemics. Influenza A viruses are further classified by surface proteins called hemagglutinin (H) and neuraminidase (N), leading to designations like H1N1 or H3N2. Type A viruses can infect various animal species and have pandemic potential, while Type B viruses circulate only among humans and cause less severe illness on average.

How Influenza Spreads

The flu virus spreads primarily through respiratory droplets produced when an infected person coughs, sneezes, laughs, or speaks. These droplets can travel up to 2 meters (6 feet) and land in the mouths or noses of nearby people. You can also become infected by touching a surface contaminated with the virus and then touching your face, though this is less common. The virus can survive on hard surfaces for up to 48 hours.

Crowded, poorly ventilated spaces increase transmission risk significantly. Indoor environments during winter months create ideal conditions for viral spread, which partly explains the seasonal pattern of flu outbreaks. The incubation period—the time from infection to symptom onset—ranges from 1 to 3 days, though it can extend to 4 days in some cases.

What Are the Symptoms of the Flu?

Flu symptoms typically appear suddenly and include high fever (39-40°C/102-104°F), severe body aches, dry cough, intense fatigue, headache, sore throat, and chills. Unlike cold symptoms that develop gradually, flu symptoms hit rapidly and are more severe. Fever usually lasts 3-5 days, but cough and fatigue can persist for 1-2 weeks.

The hallmark of influenza is its sudden onset. Many people can pinpoint the exact hour they started feeling ill. This rapid symptom development distinguishes the flu from the common cold, which typically begins with mild symptoms that gradually worsen over several days. Understanding this pattern helps in early recognition and timely treatment.

The severity of flu symptoms varies considerably between individuals. While some people experience a full spectrum of debilitating symptoms, others may have milder illness that resembles a bad cold. Factors influencing symptom severity include the specific virus strain, the individual's immune status, age, and underlying health conditions. Even healthy adults can experience severe symptoms that confine them to bed for several days.

The systemic nature of flu symptoms—meaning they affect the whole body rather than just the respiratory system—reflects the body's strong immune response to the infection. The fever, body aches, and extreme fatigue result from inflammatory chemicals called cytokines that your immune system releases to fight the virus. This explains why you feel so much worse with the flu than with a typical cold.

Common Flu Symptoms in Adults

• High fever: Temperature typically reaches 39-40°C (102-104°F) and may last up to one week
• Dry cough: Persistent and often painful, can last 2-3 weeks
• Severe body aches: Muscles and joints feel intensely sore, especially in legs and back
• Extreme fatigue: Profound exhaustion that can persist for weeks after other symptoms resolve
• Headache: Often frontal, can be severe
• Sore throat: Usually develops within the first day
• Chills and sweating: Alternating sensations as body temperature fluctuates
• Nasal congestion: Less prominent than with colds but often present

Flu Symptoms in Children

Children may present differently than adults with influenza. Young children, especially those under 5 years old, may experience gastrointestinal symptoms including nausea, vomiting, diarrhea, and abdominal pain that are less common in adults. Infants may simply appear irritable, feed poorly, and have less energy than usual. Children typically have higher fevers than adults and are more prone to febrile seizures.

Ear infections (otitis media) are a common complication in children with influenza. Parents should watch for signs of ear pain, persistent crying, or tugging at ears. Croup—a condition causing a characteristic barking cough and breathing difficulties—can also occur in young children with flu.

How Long Is the Flu Contagious?

You can spread the flu before you even know you're sick. Infected people are contagious from about 1 day before symptoms appear until 5-7 days after becoming ill. You're most contagious during the first 3-4 days of illness. Children and immunocompromised individuals may remain contagious longer.

Understanding the contagious period of influenza is crucial for preventing spread to others. The fact that you can transmit the virus before experiencing symptoms makes flu particularly challenging to contain. This pre-symptomatic transmission period typically lasts about 24 hours before symptoms develop, during which time you may unknowingly expose many people to the virus.

Peak viral shedding—and therefore peak contagiousness—occurs during the first few days of illness when symptoms are most severe. As your immune system begins to control the infection and symptoms improve, viral shedding decreases. Most adults are no longer contagious once their fever has resolved for at least 24 hours without the use of fever-reducing medications.

Certain populations shed the virus for longer periods and remain contagious beyond the typical timeframe. Young children may spread the flu for up to 10 days or longer. People with weakened immune systems—due to conditions like HIV/AIDS, cancer treatment, or immunosuppressive medications—can remain contagious for weeks or even months. Healthcare providers should advise these individuals about appropriate isolation measures.

Reducing Transmission Risk

To protect others while you're sick, stay home until at least 24 hours after your fever resolves (without fever-reducing medication). Cover your mouth and nose when coughing or sneezing, preferably with a tissue or your elbow. Wash hands frequently with soap and water for at least 20 seconds. Avoid close contact with household members when possible, and consider wearing a mask if you must interact with others.

Who Is at Higher Risk for Flu Complications?

High-risk groups for serious flu complications include adults 65 and older, children under 5 (especially under 2), pregnant women, and people with chronic conditions like asthma, diabetes, heart disease, or weakened immune systems. These individuals are more likely to develop pneumonia, require hospitalization, or die from influenza.

While influenza can make anyone seriously ill, certain groups face significantly higher risks of developing severe illness and life-threatening complications. Age is a major factor: the very young and the elderly have less robust immune responses to the virus. Adults 65 and older account for 70-85% of seasonal flu-related deaths and 50-70% of flu-related hospitalizations in typical years.

Children under 5 years old, particularly those younger than 2, are at high risk because their immune systems are still developing. They're more likely to develop complications like pneumonia, dehydration, and brain swelling (encephalopathy). Children with neurological conditions or developmental delays face especially elevated risks because they may have difficulty clearing respiratory secretions or communicating their symptoms.

Chronic medical conditions significantly increase flu complications risk. The list includes respiratory diseases (asthma, COPD), heart disease, kidney disease, liver disease, diabetes, obesity (BMI ≥40), and blood disorders. These conditions either weaken the immune response, compromise organ function, or both. People with weakened immune systems—whether from disease, medication, or treatments like chemotherapy—face the highest risks.

Pregnancy and Influenza

Pregnant women are at increased risk of severe flu illness, hospitalization, and even death. The physiological changes of pregnancy—including altered immune function, increased heart rate, and reduced lung capacity—make it harder to fight respiratory infections. Influenza during pregnancy can also harm the developing baby, increasing risks of preterm birth, low birth weight, and other complications. However, the flu virus does not cross the placenta to directly infect the baby.

Flu vaccination during pregnancy is safe and strongly recommended. It protects both the mother and the baby—antibodies from the mother cross the placenta and provide the newborn with protection during the first few months of life when they're too young to be vaccinated themselves.

When Should You See a Doctor for the Flu?

Seek medical care if you have high fever lasting more than 4 days, difficulty breathing, chest pain, confusion, severe vomiting, or if symptoms improve then suddenly worsen. People in high-risk groups should contact their doctor early in illness—antiviral medications work best when started within 48 hours of symptoms.

Most healthy people with influenza can recover at home with rest, fluids, and over-the-counter medications. However, certain situations warrant medical evaluation. Recognizing when home care is insufficient can prevent serious complications and potentially save lives. The key is understanding which symptoms indicate the infection is becoming severe or that complications may be developing.

A fever that persists beyond 4 days, or one that goes away and then returns, suggests the infection isn't resolving normally or that a secondary bacterial infection may have developed. While flu fever typically peaks in the first few days and gradually improves, persistent or recurring fever is a warning sign that shouldn't be ignored.

Breathing difficulties are particularly concerning because they may indicate pneumonia—the most serious common complication of influenza. Any sensation of breathlessness at rest, rapid breathing, or difficulty completing sentences due to shortness of breath requires prompt medical attention. Chest pain with breathing can indicate pneumonia or pleurisy (inflammation of the lung lining).

High-Risk Groups Should Act Early

If you're in a high-risk group, don't wait for symptoms to become severe before seeking care. Contact your healthcare provider early in your illness—within the first 24-48 hours if possible. Antiviral medications are most effective when started early, and your doctor may recommend treatment even if your symptoms seem mild. Some high-risk individuals may benefit from having rapid influenza tests available at home for faster diagnosis.

When to Call Emergency Services

Call your local emergency number immediately if you or someone you're caring for experiences severe difficulty breathing, bluish discoloration of the lips or face, persistent chest pain, confusion or altered mental status, seizures, or inability to wake up or stay alert. These symptoms can indicate life-threatening complications requiring emergency intervention.

How Is the Flu Treated?

Flu treatment includes rest, fluids, and medications to relieve symptoms like fever and pain. Antiviral medications (oseltamivir/Tamiflu, zanamivir/Relenza, baloxavir/Xofluza) can shorten illness duration and reduce complications when started within 48 hours of symptoms. Antibiotics do not work against flu since it's caused by a virus, not bacteria.

The cornerstone of influenza treatment for most people is supportive care—measures that help your body fight the infection and relieve symptoms while your immune system does the work of clearing the virus. This includes adequate rest, staying well-hydrated, and using over-the-counter medications to manage fever, aches, and other symptoms. For many healthy individuals, these measures are sufficient for recovery.

Rest is more than just comfort—it's therapeutic. During sleep and rest, your body diverts energy to immune function and repair processes. Fighting influenza is exhausting work for your immune system, and pushing through illness without adequate rest can prolong recovery. Most people need 1-2 weeks before they can resume normal activities, and rushing back too soon often leads to setbacks.

Hydration is critical because fever increases fluid loss through sweating, and respiratory illness increases fluid loss through breathing. Dehydration makes you feel worse and can impair immune function. Aim for clear fluids like water, herbal tea, broth, or electrolyte drinks. If you have nausea or vomiting, take small, frequent sips rather than drinking large amounts at once.

Antiviral Medications

Antiviral drugs are prescription medications that can shorten the duration of flu illness by 1-2 days and reduce the risk of complications. They work best when started within 48 hours of symptom onset, though they may still provide benefit when started later in high-risk patients. Your doctor may recommend antivirals if you're in a high-risk group or have severe illness.

Three antiviral medications are commonly used for influenza: oseltamivir (Tamiflu), taken orally twice daily for 5 days; zanamivir (Relenza), inhaled twice daily for 5 days; and baloxavir marboxil (Xofluza), taken as a single oral dose. The choice depends on factors including age, medical conditions, virus strain, and availability. Oseltamivir is the most widely used and can be given to infants as young as 2 weeks old.

Over-the-Counter Symptom Relief

Paracetamol (acetaminophen) and ibuprofen can reduce fever and relieve body aches and headaches. Follow dosing instructions carefully and don't exceed recommended doses. For adults, these medications can be alternated for better symptom control, but this approach isn't recommended for children without medical guidance. Never give aspirin to children or teenagers with flu due to the risk of Reye's syndrome, a rare but serious condition.

Decongestant nasal sprays can relieve nasal congestion but shouldn't be used for more than 3 consecutive days to avoid rebound congestion. Saline nasal drops or sprays are safe alternatives for long-term use. Cough suppressants and expectorants have limited evidence of effectiveness but may provide relief for some people. Throat lozenges, warm liquids, and honey (for children over 1 year) can soothe sore throats and coughs.

What Can You Do at Home for Flu Recovery?

Home care for flu includes bed rest, drinking plenty of fluids, using over-the-counter medications for symptom relief, keeping the room cool, and sleeping with your head elevated. Move around periodically to prevent complications. Most people recover within 1-2 weeks with proper self-care.

Effective home management can significantly impact your flu recovery experience. The goal is to support your body's healing process while minimizing discomfort. A systematic approach to self-care addresses each major symptom while allowing adequate rest for recovery. Understanding what helps—and what doesn't—can make the difference between a miserable experience and a more manageable illness.

Alternate between rest and gentle movement throughout the day. While bed rest is important, prolonged immobility can increase the risk of blood clots and prolong recovery. Every few hours, get up and move around, even if just for a few minutes. Change positions frequently while resting. When you start feeling better, gradually increase activity levels—but listen to your body and don't push too hard too soon.

Creating a Comfortable Recovery Environment

Keep your sleeping environment cool—around 18-20°C (65-68°F)—as this promotes better sleep and helps manage fever. Use light bedding that can be easily adjusted as your temperature fluctuates. A humidifier can add moisture to the air, which soothes irritated airways and makes breathing easier. Keep water and supplies within easy reach so you don't have to get up unnecessarily.

Sleep with your head elevated using extra pillows or by raising the head of your bed. This position helps with nasal congestion and makes breathing easier, particularly if you have a cough. For children, a semi-reclined position in a baby seat or being held upright may be more comfortable than lying flat.

Nutrition During Illness

Appetite loss is common with the flu, but try to eat small amounts of nutritious food when possible. Clear broths, soups, and light foods are easier to tolerate than heavy meals. The old advice about "feeding a cold, starving a fever" is a myth—your body needs nutrients to fight infection regardless of your symptoms. Focus on foods you find palatable and don't force yourself to eat if you feel nauseated.

What Complications Can Influenza Cause?

Flu complications include pneumonia (viral or bacterial), bronchitis, sinusitis, and ear infections. Serious complications like myocarditis (heart inflammation), encephalitis (brain inflammation), and multi-organ failure can occur but are rare. Flu can also worsen existing chronic conditions like asthma, COPD, or heart failure.

Pneumonia is the most common serious complication of influenza and accounts for most flu-related deaths. It can develop in two ways: primary viral pneumonia occurs when the influenza virus itself infects the lungs, while secondary bacterial pneumonia develops when bacteria invade lung tissue damaged by the flu virus. Secondary bacterial pneumonia often causes a "second wave" of illness—improvement followed by worsening fever, cough, and breathing difficulty.

Upper respiratory complications are common but usually less serious. Sinusitis develops when inflammation from the flu blocks sinus drainage, allowing bacterial infection. Symptoms include facial pain, prolonged nasal discharge, and fever. Ear infections (otitis media) frequently occur in children with flu and cause ear pain, fever, and irritability. While uncomfortable, these complications usually respond well to treatment.

Bronchitis—inflammation of the bronchial tubes—commonly accompanies flu and is the main cause of prolonged coughing. The cough can persist for 2-3 weeks after other symptoms resolve. People with underlying lung conditions like asthma or COPD may experience severe exacerbations triggered by influenza, sometimes requiring hospitalization.

Rare but Serious Complications

Myocarditis (heart inflammation), pericarditis (inflammation around the heart), and myositis (muscle inflammation) are uncommon but serious complications. These can cause chest pain, heart rhythm abnormalities, and muscle weakness. Encephalitis (brain inflammation) and acute encephalopathy are rare neurological complications that can cause confusion, seizures, and altered consciousness. These serious complications require immediate medical attention.

In children, influenza can cause muscle inflammation leading to severe leg pain and difficulty walking. This typically occurs during recovery and resolves within a few days. Febrile seizures—convulsions triggered by rapid temperature rise—can occur in young children with flu. While frightening for parents, simple febrile seizures are usually brief and don't cause lasting harm.

How Can You Prevent the Flu?

The best flu prevention is annual vaccination, which reduces infection risk by 40-60% and significantly reduces severity if you do get sick. Additional prevention measures include frequent handwashing, avoiding touching your face, staying away from sick people, and maintaining good ventilation in indoor spaces.

Prevention is always preferable to treatment when it comes to influenza. While no prevention method is 100% effective, combining vaccination with good hygiene practices significantly reduces your risk of getting the flu and spreading it to others. For high-risk individuals, prevention is especially critical because the consequences of infection can be severe.

Hand hygiene is your first line of defense against flu transmission. Wash hands frequently with soap and water for at least 20 seconds, especially after being in public spaces, before eating, and after touching your face. When soap and water aren't available, use alcohol-based hand sanitizer with at least 60% alcohol content. Make hand hygiene a habit, not just something you do when you remember.

Avoid touching your eyes, nose, and mouth—the entry points for respiratory viruses. This is challenging because the average person touches their face 16-23 times per hour, often unconsciously. Being mindful of this tendency and making an effort to reduce face-touching can decrease infection risk. Keep tissues handy and use them to touch your face when necessary.

Environmental Prevention Measures

Maintain distance from people who are visibly ill when possible. In crowded settings during flu season, consider wearing a mask, especially if you're in a high-risk group. Good ventilation reduces airborne virus concentration—open windows when possible and avoid crowded, poorly ventilated spaces during peak flu season. Outdoor gatherings carry lower transmission risk than indoor ones.

Regularly clean and disinfect frequently touched surfaces in your home and workplace. Door handles, light switches, phones, keyboards, and countertops can harbor flu viruses for up to 48 hours. Standard household disinfectants are effective against influenza viruses. Pay particular attention to cleaning when someone in your household is sick.

How Effective Is the Flu Vaccine?

Flu vaccine effectiveness varies from 40-60% depending on how well the vaccine matches circulating strains. Even when the match isn't perfect, vaccination reduces illness severity, hospitalization by 40-70%, and death from flu. Because flu viruses constantly change, annual vaccination is needed. Vaccination is especially important for high-risk groups.

Influenza vaccination is the single most effective way to prevent flu and its complications. While the vaccine isn't perfect, its benefits are substantial and well-documented. Understanding how the vaccine works and why its effectiveness varies helps set appropriate expectations while recognizing the genuine protection it provides.

Each year's flu vaccine is formulated to protect against the three or four influenza viruses that research indicates will be most common during the upcoming season. This prediction is made months in advance to allow time for vaccine production. When the prediction closely matches the actual circulating viruses, vaccine effectiveness is higher. When there's a mismatch, effectiveness may be lower—but even then, vaccination provides meaningful protection.

Beyond preventing infection, vaccination reduces illness severity in people who do get the flu despite being vaccinated. Vaccinated individuals who contract influenza typically have milder symptoms, shorter illness duration, and lower risk of serious complications compared to unvaccinated people. Studies consistently show vaccination reduces flu-related hospitalizations by 40-70% and significantly reduces mortality.

Who Should Get Vaccinated?

The World Health Organization and most national health authorities recommend annual flu vaccination for everyone 6 months and older, with special emphasis on high-risk groups. Priority groups include adults 65 and older, children under 5, pregnant women, people with chronic medical conditions, healthcare workers, and caregivers of high-risk individuals. Getting vaccinated also helps protect those around you who can't be vaccinated or may not respond well to the vaccine.

The best time to get vaccinated is in autumn, before flu season begins. It takes about two weeks after vaccination for antibodies to develop and provide protection. However, vaccination later in the season still offers benefit—flu activity can continue into spring, and it's better to get vaccinated late than not at all.

Vaccine Safety

Flu vaccines have an excellent safety record based on decades of use in hundreds of millions of people. Common side effects are mild and temporary: soreness at the injection site, low-grade fever, and mild aches. Serious side effects are very rare. The flu vaccine cannot give you the flu—inactivated vaccines contain killed virus, and the nasal spray vaccine contains weakened virus that cannot replicate effectively in the body.