Is the Flu Viral or Bacterial?

The flu (influenza) is a common and contagious illness caused by a virus. It can cause symptoms like fever, cough, and body aches. Most respiratory infections — like the flu, the common cold, and COVID-19 — are viral. But some are caused by bacteria, such as strep throat or certain types of pneumonia.

Because these illnesses can look and feel so similar, it’s easy to wonder: Is the flu viral or bacterial? For instance, both the flu and strep throat can cause a sore throat and fever.

Knowing the answer helps you understand what treatments can (and can’t) help. For example, antibiotics only work against bacteria, not viruses. 

Here, we’ll break down the key differences between viral and bacterial infections, examples of each, and why it matters when you’re deciding whether to rest, test, or get treatment.

What’s the difference between a virus and bacteria?

Bacteria and viruses are both types of germs (or microbes) that can cause infections in people. But they’re very different in many other ways:

• Viruses are much smaller than bacteria and don’t have all the features characteristics of a living organism. Bacteria are tiny living organisms, each made up of a single cell. 

• Viruses infect by entering healthy cells, using those cells to make copies of itself, and then killing the infected cells. Bacteria can make you sick without entering cells. They can multiply in tissues or produce toxins that make you sick.

• Viruses can survive on surfaces for a short time, but they can’t reproduce or grow outside of a living cell. Bacteria, on the other hand, can thrive both inside and outside of the body. 

• The immune system is able to fight both bacteria and viruses, but it has different methods for each.

• Antibiotics can kill bacteria, but they won’t work against viruses. For some viral infections, antiviral medications can shorten the illness. 

Some illnesses — like sinus infections or pneumonia — can be caused by either, so getting the right diagnosis is key. Because their symptoms often overlap, it can be hard to tell whether an infection is viral or bacterial. So getting the right diagnosis (and the right treatment) is key. 

Here’s a quick comparison of viruses and bacteria. 

What are common viral infections?

Beyond the flu, here’s a list of some familiar viral infections:

• COVID, caused by the coronavirus SARS-CoV-2

• Common cold, caused by rhinovirus, respiratory syncytial virus (RSV), and others

• Gastroenteritis (stomach flu), caused by norovirus and others

• Acute bronchitis (chest cold)

• Measles

• Chickenpox and shingles, caused by the varicella-zoster virus

• Herpes virus, caused by herpes simplex viruses 1 and 2

• Hepatitis A, B, or C

• Human papillomavirus (HPV)

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What are common bacterial infections?

Here’s a list of some familiar bacterial infections:

• Strep throat, caused by group A Streptococcus

• Pneumonia, caused by Streptococcus pneumoniae (S. pneumoniae) and others

• Ear infection, caused by S. pneumoniae and others

• Urinary tract infection (UTI), caused by E.coli and others

• Gonorrhea, caused by Neisseria gonorrhoeae

• Chlamydia, caused by Chlamydia trachomatis

• Food poisoning

• Bacterial vaginosis 

Antibiotics vs. antivirals

Antibiotics work for many bacterial infections. But they don’t fight viruses — so they won’t help you if you have the flu. 

Many antibiotics work by keeping bacteria from multiplying. Antiviral medications are different. Antivirals are designed to interfere with how a virus attaches to a healthy cell that it’s trying to infect. The result is a different type of medication.

This treatment difference is one reason why it’s important to figure out if you have a viral or bacterial infection when you get sick. This will help you get the right treatment. 

How to protect yourself from the flu and other infections

Getting a vaccine is the most effective way to protect yourself from any infection. In the case of the flu, safe and effective vaccines are recommended for most people every year. Getting a yearly flu shot is the best way to prevent the flu. 

Effective vaccines are also available against COVID and other viruses and bacteria.

Here are additional steps you can take to protect against the flu and other infections:

• Wash your hands regularly.

• Avoid being near sick people.

• Stay home if you’re sick.

• Avoid touching your eyes, nose, and mouth.

• Cover your mouth and nose when you sneeze or cough. 

• Wear a mask indoors in public areas where there are high levels of COVID. 

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Written by:

Maria Robinson, MD, MBA

Maria Robinson, MD, MBA, is a board-certified dermatologist and dermatopathologist who has practiced dermatology and dermatopathology for over 10 years across private practice, academic, and telehealth settings. She is a fellow of the American Academy of Dermatology and the American Society of Dermatopathology.

Edited by:

Sophie Vergnaud, MD

Sophie Vergnaud, MD, is the Senior Medical Director for GoodRx Health. A pulmonologist and hospitalist, she practiced and taught clinical medicine at hospitals in London for a decade before entering a career in health education and technology.

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