Influenza (the flu) and infectious mononucleosis (mono) are common illnesses that share many overlapping symptoms, but these two illnesses have different causes and require different treatments. It is easy to mistake one for the other. Therefore, getting an accurate diagnosis is important to ensuring that you’re treating your illness properly.

Causes

Both the flu and mono are caused by viruses, but the viruses differ and are transmitted in different ways.

Flu Causes

Of the four types of viruses that can cause the flu (influenza A, B, C, and D), influenza A and B are the most commonly spread from person to person. They are also the viruses responsible for the seasonal flu each year.

  • Influenza A and B

  • Transmitted through droplets from the nose, mouth, throat

  • Epstein-Barr virus (EBV)

  • Transmitted through saliva and other bodily fluids (such as blood and semen)

Influenza A viruses are categorized by subtypes based on proteins found on the surface of the virus, while influenza B viruses are categorized by their lineage. Influenza B viruses tend to change more slowly than influenza A viruses, meaning they are slower to create new variants.

One way you can catch the flu is if someone infected with influenza will sneeze or cough directly onto an object or surface right before you touch it. Still, this is why frequent hand washing is one of the preventive measures against the flu.

Mono Causes

Mono is usually caused by the Epstein-Barr virus (EBV).

Click Play to Learn About the Symptoms of the Flu

This video has been medically reviewed by Sameena Zahoor, MD

However, other viruses can also cause this disease, including:

  • Cytomegalovirus (CMV)
  • Human immunodeficiency virus (HIV)
  • Rubella, also called German measles
  • Hepatitis A, B, or C
  • Adenovirus, which is a common cause of colds

These viruses are commonly spread amongst teenagers and young adults. In fact, one of every four teenagers and young adults is infected by EBV. Viruses that cause mono can also be spread via blood transfusions, organ transplantations, and through blood and semen during sexual contact.

Symptoms

Even though the flu and mono have similar symptoms, you can tell which illness you have by when your symptoms first began and how long they last.

Flu Symptoms

People with the flu will often experience some or all symptoms within two days of being exposed to an influenza virus. People with the flu are most contagious (most likely to spread the flu) three or four days after their illness starts.

  • Fever

  • Cough

  • Sore throat

  • Runny or stuffy nose

  • Muscle or body aches

  • Headaches

  • Fatigue

  • Vomiting and diarrhea

  • Extreme fatigue

  • Fever

  • Sore throat

  • Head and body aches

  • Swollen lymph nodes in neck and armpits

  • Rash

In uncomplicated cases, symptoms usually resolve within five to seven days. However, coughs and general feelings of illness may linger for up to two weeks after the infection.

Fever is not present in all cases of the flu, and should not be a determining factor to measure your level of sickness.

Mono Symptoms

While symptoms of the flu generally appear soon after an infection, mono symptoms can present quickly or slowly. In some cases, people who have mono may not experience any symptoms until four to six weeks after you were infected. Even still, symptoms of mono may not occur all at once or even at all.

If you do experience symptoms, they typically resolve within two to four weeks. However, extreme fatigue can last longer. Occasionally, symptoms may persist for up to six months.

Complications

Knowing the possible complications allows you to know what to watch for when you or your loved one has the flu or mono.

EBV is one of the most common viruses found all over the world. In fact, most people will become infected with EBV at some point in their lives, but they may not have any symptoms.

Flu Complications

The flu is a common cause of pneumonia, especially among younger children, the elderly, pregnant women, or those with certain chronic health conditions or who live in a nursing home. Most cases of the flu never lead to pneumonia, but those that do tend to be more severe and deadly.

  • Dehydration

  • Worsening of underlying illnesses, such as diabetes, heart disease, asthma, and chronic obstructive pulmonary disease (COPD)

  • Sinus or ear infection

  • Pneumonia

  • Enlarged spleen

  • Temporary liver inflammation (hepatitis) or jaundice (yellowing of the skin and eye whites)

  • Reactivation of the virus

Certain strains can lead to specific complications. Invasive infection with Neisseria meningitidis can lead to meningococcal meningitis (the bacteria infect the lining of the brain and spinal cord and cause swelling).

Other possible complications include:

  • Inflammation of the heart (myocarditis)
  • Inflammation of the brain (encephalitis)
  • Inflammation of the muscle tissues (myositis and rhabdomyolysis)
  • Multi-organ failure, such as respiratory and kidney failure
  • Sepsis

Mono Complications

Although unpleasant, most cases of mono are not life-threatening and do not usually cause serious complications. Since this is not true in all cases, we need to know what to watch for in rare occasions too.

Complications that occur in at least 1% of mono patients include:

  • Airway obstruction because of severe tonsil swelling
  • Streptococcal pharyngitis (strep throat)
  • Meningoencephalitis (brain infection)
  • Hemolytic anemia (low red blood count cell anemia)
  • Thrombocytopenia (low platelet levels)

Treatment

We can sometimes still get sick even when we are careful about washing our hands, eating and resting well, and avoiding people who are sick with the flu or mono.

Flu treatment

Most people do not need antiviral drugs to treat the flu. If your case is complicated by other medical conditions or your symptoms are not resolving, your healthcare provider may prescribe antivirals like:

  • Rest to help body fight infection

  • Hydrate to help body flush out toxins

  • Take Tylenol (ibuprofen) to reduce fever and body aches

  • Stay home

  • Eat a light diet

  • Ask about antiviral medications if severely ill

  • Rest

  • Hydrate

  • Take NSAIDs or Tylenol (ibuprofen)

  • Stay home

  • Gargle with salt water and suck on lozenges

  • Avoid sports to prevent spleen rupture

  • Tamiflu (oseltamivir phosphate)

  • Relenza (zanamivir)

  • Rapivab (peramivir)

  • Xofluza (baloxavir marboxil)

Antiviral drugs are not the same as antibiotics, which are meant to kill bacteria. They are prescription-only medications that can:

Children and adolescents with fevers should not take aspirin. It has been associated with a rare disorder that causes brain and liver damage (Reye’s syndrome).

  • Treat the fluMake your illness less severe and shorter in durationHelp prevent serious flu complications

They are most effective when taken early in the illness (within two days of getting sick). But you can still talk to your healthcare provider about them even if this period has passed.

Mono Treatment

You treat mono at home with rest and plenty of fluids as well. Antibiotics and antiviral medications do not work for mono.

The Best Treatment for the Flu Is Prevention

Protecting yourself against several influenza A and B strains at once means getting the seasonal flu shot. Studies have shown that flu shots reduce flu illness risk between 40% and 60% during flu season. 

You should still call your healthcare provider or seek medical help if you have any of the following:

  • Difficulty breathing or swallowing
  • Dizziness or fainting
  • Extreme muscle weakness in your arms or legs
  • Intense body aches
  • Persistent high fever
  • Severe headaches
  • Sharp pain in the upper left abdomen

A Word From Verywell 

No one likes being sick, but knowing what’s wrong and what to do makes your illness a lot more manageable. If you are unsure about the cause of your symptoms or the proper course of treatment, contact your healthcare provider for an accurate diagnosis and advice. Likewise, if you are concerned about underlying illnesses and potential complications, talk to your healthcare provider. While most people do recover from the flu and mono at home by resting, hydrating, and managing symptoms without needing medical interventions, some cases may be more severe and require medical assistance.