What Is a Flu-Like Illness?

A flu-like illness is a condition that causes symptoms similar to influenza. The 2011 clinical definition from the World Health Organization (WHO) is an acute respiratory illness with a measured temperature equal to or greater than 100.4 degrees F and cough, with onset within the past 10 days. There are many potential causes, from strep throat to mononucleosis.

Flu-Like Symptoms

The 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 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 2011 World Health Organization clinical definition of flu-like illness removed the mention of sore throat that was in the previous guidelines.

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.

Potential Causes

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

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.

As well, 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.

Common Cold

Colds are caused by many different types of 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.

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 display 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.

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 the flu virus, 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, it also often displays and 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.

Coronavirus

Coronaviruses are a large family of viruses found in animals and humans. Those that infect humans can cause illness ranging from the common cold to rarer, more severe diseases such as COVID-19, 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, in rare cases, death.

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

Acute HIV Infection

The initial infection with human immunodeficiency virus (HIV) can show 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.

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. There are no vaccines, medications, or supplements that can prevent general flu-like illnesses. 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.

A Word From Verywell

There are many viruses that can make you sick and cause flu-like symptoms. If you have concerns that you may have the flu, contact your healthcare provider. This will ensure you get the appropriate care, as well as testing that will allow them to track the spread of illness in your community.