Flu-Like Symptoms and Their Causes

Flu-like symptoms include fever, cough, congestion, body aches, headache, fatigue, and sore throat. These symptoms are common in influenza, but other illnesses such as the common cold, strep throat, or COVID-19 can have similar symptoms.

The World Health Organization (WHO) defines "flu-like illness" as an acute respiratory illness with a fever equal to or greater than 100.4 degrees F, cough, and onset within the past 10 days.

This article discusses flu-like symptoms and some of their possible causes. It also explores treatment and prevention of various flu-like illnesses.

What Are Flu-Like Symptoms?

Common symptoms of the flu include:

• Fever or feeling feverish, perhaps with chills
• Cough
• Sore throat*
• Runny nose or congestion
• Muscle aches or body aches
• Headaches
• Feeling tired or mentally and physically fatigued
• In children, there may be vomiting and diarrhea

*Sore throat is a common but not guaranteed symptom of flu and flu-like illness. The World Health Organization's clinical definition of flu-like illness no longer includes sore throat.

If you come down with any of these symptoms, you may assume that you have the flu—and you might be right. However, if you have any of these symptoms and your flu test is negative, you may have a flu-like illness.

Given that such illnesses are treated differently and can have a wide range of effects, some serious, it is important to be evaluated and properly diagnosed by your healthcare professional.

What Conditions Can Cause Flu-Like Symptoms?

Aside from the flu itself, there are many possible causes of flu-like symptoms.

The Common Cold

Colds are caused by many different types of flu-like viruses, such as rhinovirus, adenovirus, human coronavirus, human parainfluenza viruses, respiratory syncytial virus (RSV), and human metapneumovirus. These may produce flu-like symptoms.

Though many cold and flu symptoms overlap, including sore throat and congestion, cold symptoms tend to be milder than symptoms of the flu. Another difference between the two is how quickly the symptoms come on. While a cold gradually progresses symptom by symptom, flu symptoms come on all at once.

Because it is easy to confuse the symptoms of colds and influenza, the Centers for Disease Control and Prevention (CDC) recommends that those at higher risk for influenza complications contact their healthcare provider to determine which they may have.

COVID-19

COVID-19 can cause flu-like symptoms, many of which are hard to distinguish from influenza or even the common cold. Some people with COVID-19 have mild symptoms like a cough and runny nose, while others can have more severe symptoms resembling influenza, such as a more severe cough with fever and body aches.

COVID-19 may also cause symptoms not usually experienced with a cold or the flu, such as shortness of breath and loss of taste and smell. The best way to tell if you have COVID-19 is to take a COVID-19 test. These tests are available for at-home use, or you can get tested by a healthcare provider.

Strep Throat

Strep throat is a bacterial streptococcal infection that causes inflammation and intense throat pain. It shares many symptoms with the flu beyond sore throat, including fever, chills, and muscle aches. However, strep throat doesn't usually include cough and nasal congestion.

Only a rapid antigen strep throat test or throat culture can diagnose strep. As this is a bacterial infection, rather than viral one, it is treated with antibiotics.

Flu Vaccine

If you had the influenza vaccine (by shot or nasal spray), you may have some mild flu-like symptoms for a short period. This is because your body is mounting the desired immune system response to the vaccine.

Also, if you get the nasal spray vaccine, it contains a live, attenuated virus that may produce more symptoms, such as a runny nose, sore throat, or cough.

Bronchitis

Bronchitis is the inflammation of the airways leading to the lungs. Viral infections, such as the cold or flu, are the usual causes of acute bronchitis.

Although bronchitis symptoms are not contagious, their underlying causes (like a cold or the flu), can be transmitted from one person to another.

Viral Pneumonia

Viral pneumonia is an infection that causes inflammation in the lungs. It can be caused by influenza and flu-like viruses such as RSV, and human parainfluenza virus, among others.

Early symptoms are a lot like flu symptoms—high fever, severe coughing, fatigue, headaches, and shaking or chills. While the flu comes on suddenly, pneumonia takes longer to develop and can be a complication of the flu.

Mononucleosis

Infectious mononucleosis, commonly referred to as mono, is a highly contagious disease typically caused by the Epstein-Barr virus (EBV). Although mono tends to bring about some of the same symptoms as the flu, including extreme fatigue, sore throat, and fever, symptoms may also include swollen lymph nodes, swollen spleen or liver, and rash.

Mono symptoms tend to last longer than the flu and be more severe. For instance, though many people start to feel better from mono within two to four weeks, symptoms of fatigue can persist for months.

Respiratory Syncytial Virus (RSV)

This common virus can cause flu-like symptoms in adults and children. These symptoms usually resemble the common cold but can become more severe in certain people, especially infants under the age of 6 months. Preventative medication is recommended for children under 2 years old and adults aged 60 years or older with underlying health conditions.

Some children at high risk of developing severe bronchiolitis may benefit from monoclonal antibodies. Synagis (palivizumab) and Beyfortus (nirsevimab) are monoclonal antibodies that can help protect certain infants and children 24 months and younger who are at high risk of serious complications from RSV during their RSV season.

Synagis and Beyfortus are not vaccines and cannot cure or treat a child diagnosed with RSV. If your child is at high risk for RSV infection, your pediatrician may discuss these options with you.

A baby with RSV may have trouble breathing and can become easily fatigued. They may have difficulty feeding. If your child develops flu-like symptoms that progress to more serious symptoms like rapid breathing or behavior changes, contact their healthcare provider right away.

Meningitis

People with meningitis may have flu-like symptoms such as fever and headache, but these symptoms tend to be much more severe than they are in other more common flu-like illnesses. Importantly, meningitis causes inflammation around the brain and spinal cord, which can cause a stiff neck. This symptom is not common with influenza and other flu-like illnesses.

Because meningitis can be deadly, it is important to seek medical help right away if you or your child develops flu-like symptoms along with a stiff neck. Meningitis typically requires hospitalization.

Other Coronaviruses

Besides COVID-19, a number of coronaviruses can cause flu-like symptoms. Many of these are responsible for the common cold, but others can cause rarer, more severe diseases such as Middle East respiratory syndrome (MERS) and severe acute respiratory syndrome (SARS).

Most human coronaviruses cause mild symptoms, including runny nose, cough, sore throat, fever, and shortness of breath. More dangerous types can lead to pneumonia and death.

Because symptoms of coronavirus overlap so much with the flu, lab tests are the only way to know for sure if what type of virus you have.

Acute HIV Infection

The initial infection with human immunodeficiency virus (HIV) can cause flu-like symptoms, including fever, chills, muscle aches, sore throat, and fatigue. In addition, there may be a rash, swollen lymph nodes, and mouth ulcers.

Although these symptoms usually resolve within a few weeks, HIV is a chronic illness. If you believe you have been exposed to HIV and you develop flu-like symptoms, it is important to get tested for HIV right away.

When to See a Healthcare Provider for Flu-Like Symptoms

The most common flu-like illnesses can usually be managed at home, as long as you are otherwise healthy. Unless you are very ill and need medical attention, it is important to stay home to avoid spreading your illness to others.

You may need to see a healthcare provider if:

• You are over the age of 65.
• You are pregnant.
• You have an underlying condition such as a seizure disorder or lung disease.
• You are immunocompromised.
• Your child has flu-like symptoms and they are under the age of 6 months.

Additionally, seek medical care right away if:

• You are having trouble breathing.
• You are experiencing dehydration as a result of severe vomiting.
• You have severe fatigue, dizziness, or weakness.
• Your symptoms haven't improved after five days.
• You get better but then start to feel sick again.
• You have pain in your chest.
  

Treatment

When you are diagnosed with influenza, your healthcare provider may be able to prescribe antiviral medications to help shorten the duration and reduce the severity of your symptoms. Unfortunately, these antiviral medications are ineffective against flu-like illnesses that are not caused by the influenza virus.

For most flu-like viral infections, treatment starts with self care. You can treat the symptoms with over-the-counter medications and wait for the virus to run its course.

Antibiotics are only needed if a bacterial cause is diagnosed, as with strep throat or bacterial pneumonia.

Prevention

Getting the yearly flu vaccine will help you avoid influenza, which is a major cause of sickness, lost time from work, and complications like bronchitis or pneumonia. You should also make sure you have received a COVID-19 vaccine and the boosters recommended by your healthcare provider.

Most other flu-like illnesses can't be prevented with vaccines, medications, or supplements. Instead, take other measures to keep yourself and others healthy:

• Washing your hands frequently, and properly, is the most effective way to prevent the spread of germs.
• When you don't have access to soap and water, hand sanitizer is effective at cleaning your hands as long as they aren't visibly soiled. Make sure you always have some with you.
• If you have a cough, cover your mouth with a tissue or cough into your elbow to reduce the number of germs you are spreading to other people.
• Stay home from work or school when you are sick so you don't pass germs on to others. Keep any trips outside the home to the minimum needed for food and health care. Avoid having visitors.

Summary

Influenza isn't the only condition that can cause symptoms such as fever, cough, runny nose, and body aches. Other conditions such as the common cold and COVID-19 can also cause these symptoms. These are referred to as "flu-like illnesses."

Most flu-like illnesses can be successfully managed at home. However, you should call a healthcare provider if your symptoms are serious, don't go away after a few days, or if you have risk factors such as older age or a lung condition.