What causes the flu? Diagnosis, prevention, and treatment

Overview: What is the flu?

The flu is a contagious upper respiratory infection caused by the influenza virus. The virus infects the nose, throat, and bronchi. In severe cases, the viral infection moves to the lungs and causes viral pneumonia, a more serious illness. Nearly everyone has had the flu at some point in their lives. Almost 1 in 10 people catch the flu during flu season. The symptoms are familiar: fever, dry cough, sore throat, headache, congestion, fatigue, muscle aches, joint pain, and just plain feeling sick—much like a cold, only worse. Unlike a cold, an influenza infection puts people at risk for serious complications like bacterial pneumonia. However, timely treatment with antiviral medications can reduce the risk of complications. The good news is that annual flu vaccines effectively prevent flu infections.

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What causes the flu?

The flu is caused by the influenza virus.

There are four types of influenza viruses, but influenza Type A and Type B are the most common causes of seasonal flu outbreaks or epidemics. Influenza A is the only cause of global flu pandemics. Both types of influenza have several variants. Some of these variants are more contagious than others and cause worse infections. 

RELATED: Influenza A vs. Influenza B: What’s the difference?

Risk factors for the flu

Everyone is at risk of contracting the flu. The only proven risk factor is age. 

According to a 2018 study, 9% of children aged 17 or under are at risk of flu infection during flu season. A slightly lower percentage of adults catch the flu each season. About 4% of adults 65 and older catch the flu.

However, not all flu infections are equal. Some flu infections can result in serious complications, particularly pneumonia. Additionally, pre-existing conditions such as lung disease, asthma, kidney disease, and heart disease can be made worse by a flu infection. For this reason, healthcare professionals encourage everyone older than six months to get the flu vaccine to decrease the infection burden so that those at risk for flu complications do not contract the flu. Risk groups for serious or even life-threatening complications caused by a flu infection include:

• Adults older than 65

• Young children (five years of age or younger)

• Residents in nursing homes or long-term care facilities

• Hospitalized patients

• People with chronic conditions such as asthma, lung disease, diabetes, chronic liver disease, stroke, heart disease, and other chronic disorders

• People with a weakened immune system

• Pregnant people 

• People with a body mass index over 40

• People of certain races, including Native Americans or Native Alaskans

RELATED: What groups are at high risk for flu complications?

Is the flu serious? When to see a doctor

A flu infection usually passes in a few days without causing major symptoms. Most symptoms can be treated at home with rest and symptom-relief medications. Some cases are more serious. Seek medical care for a flu infection if any of the risk factors for complications are present. 

Get emergency or immediate medical attention if you are experiencing:

• Difficulty breathing

• Racing heartbeats

• Chest pain

• Severe pain

• Fever greater than 103˚

• Worsening of existing medical conditions such as asthma

• Seizures

How is the flu diagnosed?

Most cases of the flu are diagnosed with a rapid influenza test and self-treated. When people go to a healthcare professional with flu symptoms, the diagnosis is suspectedbased on the symptoms, recent history, and a physical examination and confirmed with a rapid office-based influenza test.. The clinician will identify risk factors for flu complications. 

Rapid flu tests aren’t 100% accurate, so that the clinician may start treatment with an antiviral medication based on symptoms alone. 

For people with serious complications who are admitted to a hospital, physicians will use a test that more accurately identifies the influenza virus, called a PCR test. The clinicians may need a chest X-ray or CT scan to diagnose pneumonia and may perform other tests to identify other complications.

RELATED: How long does a flu test take—and can you trust the results?

Is the flu contagious?

The flu is highly contagious. As a respiratory infection, it can be spread through sneezing, coughing, and talking. However, the flu spreads mainly through contaminated surfaces such as other people’s hands, TV remotes, money, towels, and various surfaces like tables. People touch a contaminated surface, pick up the virus on their hands, and then touch their face, mouth, or nose with hands that are contaminated with live virus.

Influenza often spreads during the first week of infection and is usually contagious before symptoms start. Because the flu virus is so easily spread, people who are sick with the flu should take precautions not to infect others:

• Stay at home

• Wear a mask when near others

• Cover the mouth when coughing or sneezing

• Regular hand washing

• Use hand sanitizer regularly

RELATED: How to prevent the flu | Is the flu airborne? Learn how the flu spreads.

Is the flu curable?

The body’s immune system usually clears an influenza infection without treatment, except in rare cases. For most people in good health, the flu resolves on its own in a few days without treatment. For comfort’s sake, over-the-counter medications help relieve symptoms, including:

• Cough suppressants to decrease coughing

• Antihistamines to decrease runny nose, itchy eyes, sneezing, and postnasal drip

• Decongestants to relieve a stuffy nose

• Expectorants to relieve chest congestion

• Acetaminophen or NSAIDs such as ibuprofen to decrease fever and manage sore throat and pain

Healthcare providers may prescribe an antiviral drug specific to the flu for people at risk of complications, such as Tamiflu (oseltamivir) or Relenza (zanamivir). These drugs can reduce symptoms by about a day if given early enough. 

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How to prevent the flu

The surest way to reduce the risk of the flu is to get a flu vaccine every year. These vaccines aren’t guaranteed to prevent the flu, but they reduce the risk of contracting seasonal influenza by 40% to 60%. The vaccine also reduces the risk of severe complications by 26% and the risk of death by 31%, according to a 2021 study.

A yearly flu vaccine given in the fall and is the best way to prevent flu infections, but since it’s not guaranteed, a few common sense measures can improve the odds:

• Wash hands regularly—most people catch influenza from their own hands 

• Use hand sanitizer regularly

• Don’t touch your mouth, nose, or eyes with unwashed hands

• Avoid close contact with anyone who is ill, especially if they have the flu or another respiratory illness

• Eat a healthy diet

• Exercise regularly

• Don’t smoke or vape

RELATED: Flu shot 101: everything you need to know about getting vaccinated

FAQs about flu causes

What causes flu-like symptoms for months?

Symptoms of the flu can result from other upper respiratory tract viral infections, like a cold, strep throat, mono, or COVID-19. Some viral infections, such as mononucleosis, can cause symptoms similar to the flu, but symptoms can last for weeks or months. However, the symptoms of mononucleosis are usually markedly worse than those of influenza. 

What STD causes flu-like symptoms?

The most common STDs with flu-like symptoms are syphilis, hepatitis A, HIV, and the first episode of herpes. Syphilis has symptoms that come and go through several stages. When symptoms appear in the second stage, the most common symptom is a rash, which may not always be visible. Flu-like symptoms like a fever, a sore throat, headaches, tiredness, and body aches can also occur during secondary syphilis. Like the flu, these symptoms will resolve without treatment. Hepatitis A also causes flu-like symptoms, but usually, the symptoms are worse than the flu. Unlike syphilis, the body clears the hepatitis A infection without treatment, so symptoms go away on their own. When first infected with HIV, the virus typically causes cold or flu symptoms like sore throat, fever, headache, and muscle aches. Other common symptoms include a rash and swollen lymph nodes. Again, these symptoms go away without treatment, just as cold and flu symptoms do.

What causes keto flu?

Keto flu is not an infection. It is not even a recognized medical condition. Instead, keto flu is a term people use to describe how they feel after starting a ketogenic diet. People also call it “keto-adaptation” or “keto-induction.” As many as 54 symptoms have been associated with keto flu. Most do not have flu-like symptoms, but almost half of people with keto flu experience flu symptoms. Some of the symptoms of keto flu are typical of ketosis. Others are common symptoms of radical dietary changes. Others are typical of electrolyte imbalances. The most common home remedies for keto flu are increased sodium intake and electrolyte supplements. Keto flu, then, seems to have many contributing causes. The good news is that symptoms typically go away a few weeks after the start of the diet.

What’s next? Additional resources for people with the flu

Test and diagnostics

• Does Tamiflu work?, SingleCare

• Flu symptoms 101: Is it the flu or something else?, SingleCare

• Influenza: diagnosis and treatment, American Family Physician

Treatments

• Influenza, StatPearls

• Influenza treatments and medications, SingleCare

• People at a higher risk for flu complications, Centers for Disease Control and Prevention (CDC)

Scientific studies and clinical trials

• Does influenza vaccination attenuate the severity of breakthrough infections? A narrative review and recommendations for further research, Vaccine

• Seasonal incidence of symptomatic influenza in the United States, Clinical Infectious Diseases

More information on related health conditions

• Common cold treatments and medications, SingleCare

• Mononucleosis treatments and medications, SingleCare