Signs and Symptoms of Influenza (Flu)

Influenza, commonly known as the flu, causes symptoms like a cough, fever, sore throat, and fatigue.1 The flu is a viral infection of the respiratory tract (the lungs, voice box, throat, nose, mouth, and airways). It’s a highly contagious disease mostly caused by the viruses influenza A and B, both of which share similar symptoms.

The flu typically sets on rapidly.1 Not everyone with the flu will experience all, or even any, of the same symptoms. It’s possible to have the flu and spread it to others without realizing you have it.3 The flu can cause more severe symptoms or even be fatal in adults older than 65, infants, or people with certain health conditions.

Body Aches

A characteristic sign of the flu is pain in the muscles and joints, or myalgia. These aches occur due to your body’s immune reaction to flu viruses, which causes inflammation.

Cough

A need to cough is another common sign of the flu. Your lungs form mucus to trap the virus. Coughing helps to rid your body of this infected mucus.

Fatigue

Feeling very tired and needing more sleep is also a sign of infection. Your body expends more energy to fight off the disease. Other symptoms, such as coughing, can affect your sleep.

Fever or Chills

Feeling feverish and experiencing chills are signs of your body fighting off an infection. You may have sweats and fluctuations in temperature, with body temperatures climbing to between 100 to 104 degrees.

Headache

Another symptom often distinguishing the flu from a cold is headache, which is caused by cytokines. Your body releases these molecules as part of its natural immune reaction to the infection.

Runny or Stuffy Nose

A runny nose (rhinitis) or stuffy nose (nasal congestion) are common symptoms of respiratory infections like the flu. The soft tissues of your nasal passages can sometimes become inflamed due to infection.

Sore Throat

Not everyone who has the flu experiences a sore throat, but a dry, painful throat can sometimes be a sign of infection. It arises as the virus enters your body through soft tissues in the nasal passages and airways, causing irritation.

Less Common Symptoms

The flu can spread from the respiratory tract to other parts of the body in some more severe cases—especially if the body is not able to fight off the infection. This response can lead to several symptoms, including an ear infection, pneumonia, and sinus infection.

Ear Infection

The flu virus can spread to the passages of the middle ear and cause inflammation and an ear infection, or otitis media. Ear pain and pressure, fever, and fussiness (in infants and children) are the most common signs.

Pneumonia

Pneumonia is a severe infection characterized by fluid or pus (a viscous fluid) build-up in the air sacs, or alveoli, of the lungs. It ranges in severity and can be fatal.

Pneumonia causes symptoms like:

• Chest pain when coughing or breathing
• Chills
• Cough with or without mucus
• Fever
• Low oxygen levels in the blood (hypoxemia)
• Shortness of breath or difficulty breathing

Sinus Infection

Sinus infections (sinusitis) affect the pockets of air in your forehead, nasal passages, and cheeks. They are a moderate complication of the flu.

This causes fluid build-up in these spaces, leading to symptoms like:

• Bad breath
• Cough
• Headache
• Pain or pressure in the face
• Post-nasal drip (mucus in the throat)
• Runny or stuffy nose
• Sore throat

Rare Symptoms 

The flu can lead to life-threatening complications in rare, severe cases. Certain populations are at increased risk of these, including infants or children younger than 5, people older than 65, pregnant people, and those with health conditions like asthma, diabetes, or heart disease.

Encephalitis

The flu can cause a severe infection called encephalitis if it spreads to the tissues of the brain. This causes many symptoms, including:

• Difficulties understanding speech or speaking
• Double vision
• Flu-like symptoms
• Hallucinations
• Loss of consciousness
• Loss of touch sensation in parts of the body
• Memory loss
• Muscle weakness
• Partial paralysis of arms or limbs
• Seizures

Multiple Organ Failure

Multiple organ failure is one of the most common causes of death in people who experience flu complications. It’s characterized by multiple organs, such as the lungs or kidneys, failing to operate.

Symptoms of multiple organ failure include:

• Fatigue
• Headaches
• Itchiness
• Loss of appetite
• Memory or cognitive problems
• Pain and stiffness in the joints
• Sleep difficulties
• Swelling in the limbs
• Weight loss

Muscle Infections

Other rare complications of the flu affect the muscles. This can lead to myositis, or a cluster of conditions that cause muscle weakness, fatigue, and pain.

Myositis can lead to rhabdomyolysis (rhabdo), or a potentially fatal breakdown of the muscles. Rhabdo can cause kidney or heart failure.

Myocarditis

Myocarditis occurs when the flu spreads to the muscles of the heart. This very serious condition causes symptoms like:

• Cardiac arrest (the heart stopping)
• Chest pain
• Fatigue and loss of energy
• Irregular heartbeat
• Persistent cough
• Swelling in the arms or legs
• Thick mucus, sometimes with spots of blood
• Wheezing

Sepsis

Sepsis is a blood infection that occurs due to an extreme immune reaction in your body. It’s a dangerous medical emergency that requires immediate treatment.

Signs and symptoms of sepsis include:

• Difficulty breathing or shortness of breath
• Disorientation or confusion
• Elevated heart rate
• Fever, chills, or shivers
• Rapid breathing
• Severe pain or discomfort
• Sweating or clammy skin

Symptoms in Children

Children, especially those younger than 2, are at a higher risk of the flu and its complications. About 9.3% of those younger than 18 develop the flu, which is more than twice the rate of adults older than 65.

It’s important to know the signs and symptoms of the flu in infants and children, such as:

• Body aches
• Diarrhea
• Earaches
• Fatigue
• Fever or chills
• Nausea or vomiting
• Runny nose
• Sore throat

Is Influenza Contagious?

The flu is a highly contagious respiratory infection. It’s spread from person to person through infectious droplets. An infected person can spread these droplets when they cough, sneeze, or talk near you. The droplets can land in your mouth or nose. It’s less common, but you may pick up flu germs when you touch an infected surface and then touch your face.

When To Contact a Healthcare Provider 

Any sign of severe illness warrants medical attention, which is critical given how severe the flu can be. Certain signs and symptoms in adults warrant emergency help, such as:

• Chronic pain or pressure in the chest
• Dizziness, lightheadedness, and confusion that doesn’t go away
• A fever or cough that gets better but then returns and gets worse
• Lack of urination
• Seizures
• Severe muscle pain, weakness, or loss of balance
• Shortness of breath or breathing difficulties
• Worsening symptoms of other conditions, such as asthma

Get medical attention right away if your child or infant has:

• Any fever in infants younger than 12 weeks or a fever above 104 degrees in children older than 3 months
• Bluish tint to the face or lips
• Chest or muscular pain
• No alertness when awake, a lack of interaction with others
• Rapid or difficult breathing
• Ribs retracting with every breath
• Seizure
• Signs of dehydration, such as dry mouth, lack of urination for eight hours or more, and no tears when crying
• Worsening fever or cough
• Worsening symptoms of other conditions

Questions To Ask Your Provider

You may ask a healthcare provider the following question if you seek further care:

• How can I prevent severe complications from the flu?
• How can I tell if I have an ear or sinus infection or pneumonia?
• Should I get tested for COVID-19?
• What should I eat or drink to feel better?