Is the Flu Viral or Bacterial?

The flu (influenza) is caused by a virus. But bacteria can cause illnesses with similar symptoms, and it can be hard to tell the difference. For example, the flu can cause a sore throat and fever, but some bacterial illnesses, like strep throat, can feel the same.

So it’s no surprise that people often wonder if their infection is caused by a virus or bacteria. Here we’ll review some of the main differences between viruses and bacteria, common illnesses caused by each, and why it’s important to know the difference.

Is the flu caused by a virus or bacteria?

The flu is a common and contagious illness caused by a virus. There are four types of flu virus, and two of them (types A and B) cause seasonal flu and flu epidemics. 

Type C only causes mild illness in people and doesn’t cause epidemics. Type D can infect cattle, but it doesn’t cause illness in people. 

What’s the difference between a virus and bacteria?

Bacteria and viruses are types of germs (or microbes) that can cause infections in people. Here are some important differences between them:

• Viruses are very small and don’t have all the characteristics of a living organism. Bacteria are tiny living organisms, each made up of a single cell. 

• Viruses infect us by entering our own healthy cells, using that cell’s own machinery to reproduce, and then killing the infected cells. Bacteria often don’t need to enter cells directly when they cause an infection.

• Viruses can remain on surfaces, but they can’t reproduce or grow outside of a living cell. Colonies of bacteria can thrive inside or outside of the body. 

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

• Bacteria can be killed by antibiotics. Antibiotics do not work against viruses. In some cases, antiviral medications are available instead.

It’s often difficult to tell whether an infection is caused by a virus or bacteria. Some conditions, like a sinus infection, can be caused by both, which can make it harder to know the cause. 

What are common viral infections?

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

• COVID-19, caused by 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)

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 and others

• Ear infection, caused by streptococcus pneumoniae and others

• Urinary tract infection (UTI), caused by Escherichia coli (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 what type of infection you have when you get sick. 

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.     

Why trust our experts?

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. An experienced and dedicated pulmonologist and hospitalist, she spent a decade practicing and teaching clinical medicine at academic hospitals throughout London before transitioning to a career in health education and health technology.

Reviewed by:

Kerry R. McGee, MD, FAAP

Kerry McGee, MD, FAAP, has over a decade of experience caring for babies, children, and teenagers as a primary care pediatrician. She has a special interest in adolescent health, particularly in adolescent mental health.

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