Is the Flu a Virus or Bacteria?

Understanding the Differences Between Bacterial and Viral Infections

Influenza (referred to as the "flu") is a contagious respiratory disease caused by the influenza A, B, or C virus.  Influenza viruses are airborne and affect the nose, throat, and lungs.

Not all illnesses are caused by viruses. Some such as bacterial pneumonia can cause flu-like symptoms but are caused by bacteria. However, the flu can lead to bacterial infections as a person's natural defenses may be weakened during an infection.

Infections that occur shortly after the primary infection are called secondary infections. In this article, we will cover the differences between bacterial and viral infections.

Woman sick with the flu

triloks / Getty Images

Is the Flu a Virus or Bacteria?

The flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. However, it is not the same as stomach flu (gastroenteritis). Stomach flu is commonly caused by rotaviruses or noroviruses, which are two groups of highly contagious viruses that infect the gastrointestinal system.

Flu Symptoms

Influenza viruses are detected year-round in the United States. However, flu viruses are most common during the fall and winter, peaking between December and February. The flu can cause mild to severe illness and, at times, can lead to death.

While flu symptoms are similar to a cold or upper respiratory tract infection, the main difference is that flu symptoms come on suddenly. Most people who contract the flu will experience some degree of fever, chills, headaches, body aches, congestion, coughing, and fatigue.

Flu symptoms vary from person to person. People who have the flu often feel some or all of these common signs and symptoms. They include:

  • Fever
  • Aching muscles
  • Chills and sweats
  • Headache
  • Dry, persistent cough
  • Shortness of breath
  • Tiredness and weakness
  • Runny or stuffy nose
  • Sore throat
  • Eye pain
  • Vomiting and diarrhea (this is more common in children than adults)

Who Is More Susceptible to the Flu?

Anyone can get sick with flu, and serious problems can occur at any age. However, people who are more susceptible to complications of the flu include those that are:

  • 65 years and older
  • People of any age with certain chronic medical conditions (such as asthma, diabetes, or heart disease)
  • People who are pregnant
  • Children younger than 5 years (especially those younger than 2 years old)

Seek medical attention immediately if your symptoms do not improve or become worse.

What's the Difference Between Bacteria and Viruses?

Bacteria and viruses are very different pathogens (tiny organisms that cause disease). However, they have some similarities.

They both cause infections and are spread by:

  • Respiratory droplets (coughing, sneezing)
  • Exposure to infected blood or body fluids (such as semen and vaginal secretions)
  • Skin-to-skin contact
  • Infected animals and insects
  • Contact with contaminated surfaces, food, and water.


Bacteria are small single-cell organisms. They have a nucleus that contains their genetic material and they can reproduce on their own. They carry out many other processes to sustain their lives.

Remarkably, less than 1% of bacteria are harmful to humans. They are found widely in nature, such as in the soil and water. You also have "good" bacteria that belong in your body and help you. These bacteria digest your food and protect your body from harmful bacteria.

But some bacteria can infect the body, causing harm. This triggers your body to mount a protective immune response.


Viruses are microscopic parasites, generally much smaller than bacteria. Unlike bacteria, viruses can't survive without a host cell. They can only reproduce by attaching themselves to cells and using the cell's processes to replicate.

Viruses can be specific about the cells they attack. For example, certain viruses attack cells in the liver, respiratory, or blood. There are also viruses that attack plants and bacteria. As with bacterial infections, the immune system responds to viral infections.

Bacterial Infections

All of the organs in our body are susceptible to bacterial infection.

How Are Bacterial Infections Transmitted?

There are many modes of transmission of bacteria. Bacteria are transmitted to humans through air, water, food, parasites, animals, or contact with other humans.

Also, bacteria that normally are present in or on the body without causing illness can produce illness if a person's immune system is weakened and they overgrow or are introduced to a different area of the body, such as the bloodstream, lungs, or urinary tract.

Common Bacterial Infections and Their Symptoms

The severity of bacterial infections can vary widely and depends on the type of bacteria involved. Some illnesses can be minor, but some can be more severe. Common respiratory bacterial infections and their symptoms include:

  • Strep throat: Streptococcus pyogenes can be spread from person to person by coughing, sneezing, and coming into contact with contaminated objects. It causes a sore throat and fever.
  • Bacterial pneumonia: Streptococcus pneumoniae is the classic bacteria causing pneumonia (infection of the lungs), although other bacteria may also be the cause. Symptoms include cough, high fever, chills, chest pain, and difficulty breathing. Bacterial pneumonia can develop after a viral upper respiratory tract infection.
  • Tuberculosis: Mycobacterium tuberculosis can infect the lungs and can spread to the kidney, brain, and spine. Symptoms include a bad cough that lasts three weeks or longer, coughing up blood, fever, weakness, weight loss, and night sweats.

Viral Infections

Like bacteria, not all viruses are considered harmful. Some can cause illness, but many simply coexist with you. Viruses are made up of genetic material (DNA or RNA) and a protein coat, which can enter host cells to cause infection.

How Are Viral Infections Transmitted?

Viruses replicate within the host’s cells to create copies that can be passed on, or transmitted, to others. Like bacteria, virus transmission can occur through multiple pathways. Some viruses can be transmitted by respiratory droplets when a person talks, coughs, or sneezes.

Viruses can be transmitted when you touch a surface that is contaminated and then touch your eyes, nose, or mouth. They can also be transmitted by contaminated water or food. Some viruses are spread through sexual contact or through contact with blood via needle or transfusion.

Common Viral Infections and Their Symptoms

The severity of viral infections can also vary widely and depends on the type of virus involved. Common viral respiratory infections and their symptoms include:

  • The common cold: Typically caused by rhinoviruses and some coronaviruses. The common cold is usually mild and can include coughing, sore throat, sneezing, runny and stuffy nose.
  • Influenza (flu): The flu is commonly caused by type A and B influenza strains. Symptoms are typically more severe than the common cold and can include coughing, fever (or chills), muscle aches, shortness of breath, and more.
  • COVID-19: Caused by the SARS-CoV-2 virus, symptoms include fever, dry cough, sore throat, runny nose, shortness of breath, loss of sense of smell and taste, fatigue, and more.
  • Viral pneumonia: This viral lung infection can be caused by viruses including coronaviruses, adenoviruses, influenza virus, parainfluenza virus, varicella-zoster virus, and respiratory syncytial virus (RSV). Symptoms include cough, difficulty breathing, increased breathing rate, and fever.

Is My Infection Bacterial or Viral?

Differentiating between the two requires medical intervention since both may cause fever and irritability. Treatments also vary significantly.


Depending on the type of bacterial or viral infection you may have, symptoms vary. For example, common viral infections such as an upper respiratory infection (common cold or flu) can typically be detected by a runny nose, cough, low-grade fever, sore throat, and difficulty sleeping.

Bacterial pneumonia, which is caused by bacteria, may result in a persistent cough, chest pain, fever, or difficulty breathing. 

Mucus Color

You might have heard that having clear or white mucus often indicates a viral infection, while yellow or green mucus may suggest a bacterial infection. However, this is a common misperception.

When you have a cold, your immune system sends white blood cells called neutrophils rushing to the area. These cells contain a greenish-colored enzyme, and in large numbers they can turn the mucus the same color.

If you are unsure whether you have a bacterial or viral infection, avoid self-diagnosis, as it may be inaccurate. Always consult a healthcare professional to make the proper diagnosis for you.

Diagnosis of Bacterial and Viral Infections

If you believe you have an infection other than the common cold, which is usually not life-threatening, consult your healthcare provider to make the proper diagnosis and treatment plan.

A healthcare provider will use your report of symptoms, medical history, and a physical examination to begin the diagnosis of a respiratory infection. Depending on possible exposures, they may refer you to testing for bacteria or viruses.

Tests that are frequently performed to diagnose respiratory bacterial infections include:

  • Rapid strep test: A throat swab is analyzed for strep throat.
  • Throat culture: A swab is taken and sent to the lab for culture.
  • Sputum culture: Phlegm is collected and sent to the lab for culture.
  • Blood culture: Blood is drawn into special bottles and incubated in the lab.

To see if you have a viral respiratory infection, commonly used laboratory tests that are used include:

  • Nasal, cheek, or throat swab: This may be used for detection of viral antigens or for viral culture for influenza or COVID-19.
  • Blood tests: Blood may be drawn to test for viral antigens or antibodies.

Treatment of Bacterial and Viral Infections

Bacterial infections (such as strep throat) are most often treated with antibiotics, which are medications that prevent bacterial growth or kill them. Antibiotics do not work on viral infections.

Self-care such as getting enough rest and fluids and using over-the-counter medications for symptoms (such as congestion or cough) may be appropriate in mild cases of the common cold and flu.

People who are at high risk for complications from influenza (such as pneumonia) may be treated by their healthcare provider with antiviral drugs.

Severe respiratory infections may require more extensive treatment, which could include hospitalization.

When to See a Doctor

Seek medical attention immediately if you suspect that you have an infection and you have experienced:

  • An animal or a human bite
  • Difficulty breathing
  • A cough lasting longer than a week
  • Periods of rapid heartbeat
  • A rash, especially if it's accompanied by a fever
  • Swelling
  • Blurred vision or other difficulty seeing
  • Persistent vomiting
  • An unusual or severe headache

Preventing Bacterial and Viral Infections

Effective ways to prevent bacterial and viral infections include the following:

  • Frequent hand washing (at least 20 seconds with soap and warm water)
  • Using alcohol-based hand sanitizers
  • Getting adequate sleep
  • Keeping yourself hydrated
  • Avoiding people who are sick and staying away from others when you are sick
  • Getting enough vitamins and minerals
  • Practicing safer sex
  • Cooking food thoroughly to prevent the risk of foodborne illness
  • Preventing bug bites with insect repellents that are approved by the Environmental Protection Agency (EPA)
  • Staying current with recommended vaccinations, including childhood vaccinations, the yearly flu vaccine, pneumonia vaccine, and COVID-19 vaccine

Preventing the Flu

To prevent viral infections such as the flu, the Centers for Disease Control and Prevention (CDC) highly recommends that people get their yearly flu vaccine, which is both safe and effective. If you aren't sure if vaccines are right for you and your family, be sure to discuss them with your healthcare provider.


Bacteria and viruses are very different pathogens (tiny organisms that cause disease). However, they have some similarities. They both cause infections and are spread by respiratory droplets (coughing, sneezing) and contact with contaminated surfaces, food, water, infected animals, and insects.

Differentiating between bacterial and viral infections requires medical intervention. Treatments also vary significantly. Bacterial infections can be treated with antibiotics while viral infections cannot. Some viral infections can be successfully treated with antiviral medications.

Common respiratory bacterial infections include strep throat and bacterial pneumonia. Common viral respiratory infections include the common cold and influenza.

A Word From Verywell

You're likely going to have at least a few bacterial and viral infections throughout your life. These infections can cause a range of symptoms and effects. Your healthcare provider can use your symptoms and diagnostic testing to determine which specific infection you have.

Sometimes, you may need prescription medicine, such as antibiotics or antivirals, to treat your condition. If you think you have a bacterial infection that needs treatment, talk to your doctor.

Frequently Asked Questions

  • Is a cold a virus or bacteria?

    The common cold is a viral infection that affects your upper respiratory system. It can be caused by many types of viruses, including rhinoviruses and some coronaviruses. Symptoms are generally mild and include coughing, sneezing, runny or stuffy nose, sore throat, and in some cases, a low-grade fever.

  • How many deaths has the flu caused?

    The CDC estimates that approximately 12,000 to 52,000 deaths from the flu have occurred annually in the United States between 2010 and 2020.

  • What is the fastest way to cure a viral infection?

    Many viral infections such as the common cold or flu can be self-managed without medical intervention. The best way to treat viral infections is to get adequate sleep, stay hydrated, and take over-the-counter medications for bothersome symptoms. If your symptoms do not improve or become worse, consult your healthcare professional as soon as possible.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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