Influenza, widely known as the flu, is a contagious viral illness that primarily affects the respiratory system, including the lungs, throat, nose, and airways. It commonly causes symptoms such as coughing, fever, sore throat, body aches, headaches, and extreme fatigue. The flu is mainly caused by influenza A and B viruses, which produce similar clinical symptoms and can spread rapidly within communities.

One defining feature of influenza is its sudden onset. Symptoms often appear abruptly and can range from mild to severe. Not everyone experiences the same combination of symptoms, and some individuals may carry and transmit the virus without realizing they are infected. This makes influenza particularly dangerous, especially for vulnerable populations such as adults over 65, infants, pregnant individuals, and people with chronic medical conditions.

Common Flu Symptoms

Body Aches and Muscle Pain
Muscle and joint pain, also known as myalgia, is a hallmark symptom of the flu. These aches result from the immune system’s inflammatory response as it works to eliminate the virus. The discomfort can be widespread and intense, often affecting daily activities.

Cough
A persistent cough is another frequent symptom. During infection, the lungs produce mucus to trap viral particles, and coughing helps expel this infected mucus. The cough may be dry or productive and can linger even after other symptoms improve.

Fatigue
Extreme tiredness is common with influenza. The body uses significant energy to fight the infection, and symptoms like coughing or fever can disrupt sleep, further worsening exhaustion. Many people report needing more rest than usual for several days or even weeks.

Fever and Chills
Fever is a classic sign of influenza and indicates that the immune system is actively fighting the virus. Body temperatures can rise between 100°F and 104°F (37.8°C–40°C), often accompanied by chills, sweating, and temperature fluctuations.

Headache
Headaches are more prominent in the flu than in the common cold. They are caused by cytokines—immune signaling molecules released during infection—which can trigger inflammation and pain in the head and neck.

Runny or Stuffy Nose
Nasal congestion or a runny nose is common due to inflammation of the nasal passages. Swollen tissues and excess mucus production contribute to breathing discomfort and sinus pressure.

Sore Throat
A sore, dry, or irritated throat may occur as the virus enters through the nasal passages and airways. While not universal, this symptom can make swallowing and speaking uncomfortable.

Less Common but Serious Complications

In more severe cases, influenza can spread beyond the respiratory tract, leading to complications that may require medical treatment or hospitalization.

Ear Infections
The virus can travel to the middle ear, causing otitis media. Symptoms include ear pain, pressure, fever, and irritability, especially in young children.

Pneumonia
Pneumonia is one of the most serious complications of the flu. It occurs when fluid or pus fills the air sacs of the lungs, impairing oxygen exchange. Symptoms include chest pain, persistent cough, fever, chills, low oxygen levels, and difficulty breathing. Pneumonia can be life-threatening, particularly in high-risk individuals.

Sinus Infections
Sinusitis may develop when mucus becomes trapped in the sinus cavities. This leads to facial pain or pressure, headaches, post-nasal drip, nasal congestion, sore throat, and bad breath.

Rare and Life-Threatening Complications

Although uncommon, severe influenza can result in critical conditions, especially in high-risk populations.

Encephalitis
If the virus spreads to the brain, it can cause encephalitis, a serious inflammation of brain tissue. Symptoms may include confusion, memory loss, hallucinations, seizures, speech difficulties, muscle weakness, or loss of consciousness.

Multiple Organ Failure
In extreme cases, influenza complications can lead to the failure of multiple organs such as the lungs, kidneys, or liver. Symptoms include severe fatigue, cognitive issues, joint pain, swelling of the limbs, sleep disturbances, and unintended weight loss.

Muscle Infections
The flu can cause myositis, leading to muscle inflammation, weakness, and pain. In rare cases, this may progress to rhabdomyolysis, a dangerous breakdown of muscle tissue that can result in kidney or heart failure.

Myocarditis
When the virus affects the heart muscle, myocarditis may develop. This condition can cause chest pain, irregular heartbeat, swelling of the limbs, fatigue, persistent cough, and in severe cases, cardiac arrest.

Sepsis
Sepsis is a medical emergency caused by an overwhelming immune response to infection. Warning signs include rapid breathing, confusion, high heart rate, fever or chills, severe pain, and clammy skin. Immediate treatment is critical to prevent death.

Flu Symptoms in Children

Children, especially those under the age of two, are more susceptible to influenza and its complications. Symptoms in children may include fever, chills, body aches, fatigue, runny nose, sore throat, ear pain, nausea, vomiting, or diarrhea. Because children may not clearly express how they feel, caregivers should closely monitor changes in behavior or alertness.

Is Influenza Contagious?

Yes, influenza is highly contagious. It spreads mainly through respiratory droplets released when an infected person coughs, sneezes, or talks. These droplets can enter the mouth or nose of nearby individuals. Less commonly, the virus can spread by touching contaminated surfaces and then touching the face.

When to Seek Medical Care

Prompt medical attention is essential if symptoms become severe or worsen. Adults should seek emergency care for chest pain, persistent dizziness or confusion, worsening fever or cough, difficulty breathing, seizures, reduced urination, or severe muscle weakness.

Children and infants require immediate medical care if they develop very high fever, breathing difficulties, bluish skin or lips, dehydration, seizures, l