Viruses vs. Bacteria: What's the Difference?

Both can make you sick, but here's what separates the two.

Photo: Design by Jo Imperio

Let's face it—we've been talking a lot about viruses for the past two pandemic years. That means, by now, you probably know that a virus is invisible to the naked eye and can cause all types of health problems.

But bacteria check those boxes too. So what's the difference between viruses and bacteria—and why is it important to understand the difference? Here's what to know about the two germs and the illnesses they can cause.

What Are Viruses and Bacteria?

Viruses and bacteria are all around us. While we can't see them with the naked eye—in fact, viruses are submicroscopic, meaning you can't even see them in the microscope, according to the National Human Genome Research Institute—they can certainly make themselves known if they infect us.

Viruses are tiny organisms made of genetic material called nucleic acid—either DNA or RNA, per the National Human Genome Research Institute—that is surrounded by a coat of protein, Charles Bailey, MD, medical director for infection prevention at Providence St. Joseph Hospital and Providence Mission Hospital in Orange County, California, tells Health.

These little germs take over cells in your body, using components of the host cells to make copies of themselves, the National Human Genome Research Institute explains. The virus multiplies, overtakes other cells, and continues to reproduce. This process can damage or kill healthy cells, leading to illness.

While a virus can't reproduce unless it's within a cell of another organism, bacteria—larger, single-celled organisms—don't need a host to reproduce and are capable of living in various types of environments, says Dr. Bailey.

The human body is actually full of bacteria—some are harmless, and some are even helpful (like by playing a part in regulating your gut health), according to the National Human Genome Research Institute. But some bacteria are bad and, like viruses, can cause illness by replicating quickly in our bodies, damaging or killing cells and even tissue itself, as explained in the National Academy of Medicine's book What You Need to Know About Infectious Disease. Many disease-causing bacteria produce toxins, which are powerful chemicals that damage cells and make you sick, according to the Mayo Clinic.

"When people wonder what the difference is between a virus and a [bacterium], it's like comparing the difference between a roach and a shark," Theresa Fioritio, MD, an infectious disease specialist and director of the Family Travel Clinic at NYU Langone Hospital—Long Island, tells Health. "There are many differences: where they live (inside vs. outside our cells), what they eat, and—probably what's most relevant to us—how to kill them." (More on that last point in a bit.)

One thing viruses and bacteria have in common is that they both have the potential to cause infections and lead to mild, moderate, or severe illness, per the CDC.

"In recent years, as well as throughout history, we have seen pandemics and epidemics caused by viruses (eg, COVID-19, influenza, smallpox, HIV, and Ebola) and bacteria (eg, plague as the cause for the Black Death of the Middle Ages)," Dr. Bailey says.

When a virus causes an illness, it is known as a viral infection. When a bacterium causes an illness, it is known as a bacterial infection.

Symptoms of Viral vs. Bacterial Infections

While bacteria and viruses are different in terms of molecular structure, they can cause infections that have similar symptoms, such as coughing, sneezing, fever, vomiting, diarrhea, fatigue, and cramping. But symptoms vary depending on the specific infection and how severe it is.

According to the Mayo Clinic, common bacterial infections include strep throat, tuberculosis, and urinary tract infections. Common viral infections include the common cold, chickenpox, and genital herpes, per the Mayo Clinic. Obviously, all affect different parts of the body and can have a wide range of symptoms and severity.

Although bacterial and viral infections are different, they can be connected. In some cases, viral respiratory infections lead to the complication of a bacterial infection. The occurrence is known as a secondary infection, and it may be caused by changes in the immune system, according to MedlinePlus.

For instance, a 2021 study in the journal Scientific Reports found that of 642 patients hospitalized with COVID-19 (a disease caused by a virus), 12.6% went on to develop a bacterial infection. And of 742 patients hospitalized for flu (an illness caused by a virus), 8.7% developed a bacterial infection. Having that secondary bacterial infection—which was commonly caused by Staphylococcus bacteria and led to acute respiratory distress—was linked to a higher chance of death. As MedlinePlus points out, you can develop bacteria-caused pneumonia even after having a virus-caused upper respiratory infection like a cold or flu.

How Viruses and Bacteria Spread

So you know that you might get sick if you're infected with a virus or bacterium, but how would you get infected in the first place? Let's break down how the germs are spread. Depending on the type, the 2016 textbook Essential Human Virology reports that viruses can spread through:

  • Skin-to-skin contact
  • Respiratory secretions like a cough or sneeze
  • Droplets when someone speaks or breathes
  • Vomit, diarrhea, urine, or feces (either through the particles in the air or if someone contaminates food with it)
  • Saliva
  • Semen or vaginal discharge
  • Blood

The International Encyclopedia of Public Health reports that most people come in contact with infection-causing bacteria through:

  • Direct contact with an infected person or animal
  • Contact with bacteria in the air or droplets
  • An insect such as a tick that has hosted on an infected person and then bites an uninfected person
  • A contaminated inanimate object such as food, water, or utensil

Treating Viral and Bacterial Infections

If you're infected with a virus or bacterium and become sick, you might need some treatment. But how viruses and bacteria respond to medication is another difference between them.

"Viruses are treated by antiviral agents while bacteria are treated by antibacterial agents (antibiotics)," says Dr. Bailey. Antivirals can't treat bacteria, and antibiotics can't treat viruses due to the different structure of the organisms.

Antivirals include Tamiflu for the flu, Remdesivir for COVID-19, and Biktarvy for HIV. Antibiotics include amoxicillin, doxycycline, and cephalexin—all of which can be used to treat a variety of bacteria-caused infections, including some of those affecting the respiratory tract, urinary tract, and skin.

"Bacteria have cell walls and internal structures that can be targeted by antibiotics to either kill the organism or interrupt its life cycle," Dr. Bailey explains. "Viruses are simpler with fewer structural targets, but since they must enter into other cells to reproduce themselves, this offers antiviral agents an opportunity to work by interfering with these elements of the viral life cycle."

There are fewer therapeutic agents available to treat viruses compared to bacterial infections. But the CDC points out that antibiotics are not actually always needed in the treatment of all bacterial infections. For instance, many bacteria-caused sinus infections and some ear infections typically get better on their own; taking antibiotics when it's not necessary provides no benefit and might even result in harmful side effects.

Protecting Yourself From Viruses and Bacteria

Based on the ways they can spread, taking measures like practicing safe sex, cleaning human-handled food like fruit and vegetables, and using personal protective equipment can decrease your risk of getting infected by a virus, according to Essential Human Virology.

Protecting yourself against infection-causing bacteria can mean taking steps like treating water so it's safe for consumption, practicing safe sex, and vaccinating your animals, per the International Encyclopedia of Public Health.

And both publications point out that personal hygiene is key for protection against both bacteria and viruses. For example, regular hand washing helps prevent illness because germs can live on your hands, according to the Centers for Disease Control and Prevention (CDC).

Vaccination is another way to protect yourself against bacteria and viruses. Many viral infections—including the flu, mumps, and polio—as well as many bacterial infections—like pertussis, diphtheria, tetanus, pneumococcal pneumonia, and meningococcal disease—can be prevented by vaccination, says Dr. Bailey. As such, the CDC recommends sticking to an immunization schedule to protect yourself against infection.

You can also get rid of viruses and bacteria by sanitizing and disinfecting objects. Here's how the US Environmental Protection Agency (EPA) makes the distinction: "Sanitizing kills bacteria on surfaces using chemicals. It is not intended to kill viruses. Disinfecting kills viruses and bacteria on surfaces using chemicals." The EPA regulates sanitizers and disinfectants so that you can be sure that what you're using is effective.

When To See a Health Care Provider

If you are feeling ill and think you might have an infection, you can go to the doctor to find out for sure. The Mayo Clinic says medical care is especially important if you think you have an infection and have also 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

If you have an infection, your doctor will be able to figure out how serious it is and whether it's a virus or bacterium causing it. To do that, they can ask for your symptom history and might run diagnostic tests like taking samples of your urine, stool or blood, or a swab from your nose or throat. The results can then help them determine how to best treat your infection.

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