Signs and Symptoms of Influenza (The Flu)

Medically reviewed by Michael Menna, DO

Influenza—the flu—is a viral infection of the respiratory tract (the lungs, voice box, throat, nose, mouth, and airways). This disease typically sets on rapidly and causes a range of symptoms, including cough, fever, sore throat, and fatigue, among others. Influenza can cause more severe symptoms or even become fatal among more vulnerable populations, such as those older than 65, infants, or those with chronic disease.

The flu is very common. It affects anywhere from 3 to 11% of the US population—millions of Americans—every year. It is a highly contagious disease that is spread through respiratory droplets in the air or on surfaces. Since it’s so widespread, let’s take a look at influenza’s signs and symptoms and learn when it’s time to call your healthcare provider or get emergency help.

Common Symptoms

Unlike those of the common cold, the symptoms of influenza occur quickly after exposure. They typically last for one to two weeks, though you may feel sick for longer. The most common symptoms include:

• Fever and/or chills: Feeling feverish and experiencing chills are signs of the body fighting off the infection. You may have sweats and fluctuations in temperature, with temperatures climbing to between 100-104 degrees.

• Cough: A need to cough is another common sign of influenza. When you’re infected with the flu, your lungs form mucus to trap the virus. Coughing helps to rid your body of this infected mucus.

• Sore throat: Not everyone who has influenza 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.

• Runny nose: As with a sore throat, the soft tissues of the nasal passages can sometimes become inflamed due to infection. Also known as rhinitis, this causes a runny or stuffy nose (nasal congestion), a common symptom of respiratory infections.

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

• Headaches: Another symptom often distinguishing influenza from the common cold is headache. This is caused by cytokines, or molecules that your body emits as part of its natural immune reaction to the infection.

• Fatigue: Feeling very tired and needing more sleep is also a sign of influenza infection. Your body expends more energy fighting off the disease, and sometimes other symptoms can affect your sleep.

Not everyone with the flu will experience all, or even any, of these symptoms. It is possible to have the flu and spread it to others without realizing that you have it.

Less Common Flu Symptoms

In some, more severe cases—and especially if the body isn’t able to fight off the infection—influenza spreads from the respiratory tract to other parts of the body. This can lead to several complications, including a sinus infection, ear infection, and pneumonia.

Sinus Infection

Sinus infections (sinusitis) are characterized by an infection of the pockets of air in your forehead, nasal passages, and cheeks, and are a moderate complication of influenza. This causes fluid buildup in these spaces, leading to:

• Headache

• Runny or stuffy nose

• Pain or pressure in the face

• Post-nasal drip (mucus in the throat)

• Sore throat

• Cough

• Bad breath

Ear Infection

If the virus spreads to the passages of the middle ear, ear infection, or otitis media, can result, causing inflammation. Ear pain and pressure, fever, and fussiness (in infants and children) are the most common signs of this condition.

Pneumonia

A severe infection, pneumonia is characterized by fluid or pus (a viscous fluid) buildup in the air sacs, or alveoli, of the lungs. It ranges in severity and can be fatal. Pneumonia causes:

• Cough with or without mucus

• Chest pain when coughing or breathing

• Chills

• Fever

• Shortness of breath or difficulty breathing

• Low oxygen levels in the blood (hypoxemia)

Rare Symptoms

In rare, severe cases, influenza can lead to life-threatening complications. Certain populations are at increased risk of these, including infants or children under the age of 5, people older than 65, pregnant people, and those with chronic health conditions, such as asthma, diabetes, or heart disease.

Myocarditis

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

• Fatigue and loss of energy

• Chest pain

• Swelling in the arms or legs

• Irregular heartbeat

• Wheezing

• Persistent cough

• Thick mucus, sometimes with spots of blood

• Cardiac arrest (the heart stopping)

Encephalitis

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

• Flu-like symptoms

• Difficulties understanding speech or speaking

• Double vision

• Hallucinations

• Loss of consciousness

• Muscle weakness

• Partial paralysis of arms or limbs

• Seizures

• Memory loss

• Loss of touch sensation in parts of the body

Muscle Infections

Other rare complications of influenza affect the muscles. This can lead to myositis, a cluster of conditions that cause muscular weakness, fatigue, and pain. Sometimes, this leads to rhabdomyolysis, a potentially fatal breakdown of the muscles that can cause kidney or heart failure.

Multiple Organ Failure

Multiple organ failure is one of the most common causes of death in people who experience influenza complications. It is characterized by multiple organs, such as the lungs or kidneys, failing to operate. Symptoms of multiple organ failure include swelling in the limbs, headaches, itchiness, sleep difficulties and fatigue, weight loss and loss of appetite, pain and stiffness in the joints, and memory or cognitive problems, among others.

Sepsis

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

• Rapid breathing

• Elevated heart rate

• Difficulty breathing or shortness of breath

• Disorientation or confusion

• Severe pain or discomfort

• Fever, chills, or shivers

• Sweating or clammy skin

Influenza Symptoms in Children

Children, especially those under the age of 2, are at a higher risk of developing influenza as well as experiencing flu complications. Researchers estimate that about 9.3% of those under the age of 18 develop this disease, more than twice the rate of those older than 65.

This being the case, it’s important to know the signs of influenza in infants and children. Many symptoms in adults also affect this population, including fever, chills, fatigue, body aches, sore throat, runny nose, and others. In addition, infants and children may experience nausea, vomiting, diarrhea, and earaches.

When to Contact a Healthcare Provider

Any sign of an influenza complication warrants medical attention, which is critical given how severe this disease can be. Certain symptoms and signs warrant emergency help. In adults, these include:

• Shortness of breath or breathing difficulties

• Chronic pain or pressure in the chest

• Dizziness, lightheadedness, and confusion that doesn’t go away

• Seizures

• Lack of urination

• Severe muscle pain, weakness, or loss of balance

• A fever or cough that gets better but then returns and gets worse

• Worsening symptoms of other conditions, such as asthma

In addition, some signs in children or infants also prompt calling 911, such as:

• Rapid or difficult breathing

• Bluish tint to the face or lips

• Ribs retracting with every breath

• Chest pains

• Muscular pain, often causing the child to not want to walk

• Dry mouth, lack of urination for 8 hours or more, and/or no tears when crying

• No alertness when awake, a lack of interaction with others

• Seizure

• Fever above 104 degrees in children older than 3 months

• Any fever in infants younger than 12 weeks

• Worsening fever or cough

• Worsening symptoms of other conditions

A Quick Review

Influenza, commonly known as the flu, is a viral infection that primarily affects the respiratory system. Typical signs of this condition include cough, runny nose, fever, body aches, fatigue, and headache, among others.

In rare and more severe cases, influenza causes complications ranging in severity from sinus or ear infection to pneumonia, endocarditis (infection of the hear), and encephalitis (an infection of the brain tissues).

Because of the risk of complications, it’s important to seek out treatment, especially if your or your child's symptoms are severe.

Frequently Asked Questions

How long does influenza last?

Typically, influenza symptoms occur within 48 hours of exposure to the virus. In most cases, you experience symptoms for 7-10 days, though some people may feel sick for two weeks or longer. Most often—and especially in absence of other health conditions or risks—influenza is a self-limiting disease, meaning the symptoms resolve on their own.

How are symptoms of the flu different than a common cold?

While there are many similarities between the symptoms of the common cold and influenza, there are key differences. Influenza has a more rapid onset and is more likely than a cold to cause fever, body aches, fatigue or muscular weakness, and headache. In turn, stuffy nose, congestions, sneezing, and sore throat can arise in both, but are more common if you have a cold.

Can you still have the flu if you don't have a fever?

Fever is often a hallmark of influenza, arising as your immune system tries to fight off the infection. However, it’s important to note that some people develop the flu without experiencing fever or chills. This is especially the case if you’re older than 65 or have a compromised or weakened immune system due to other conditions or diseases.

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