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 are tiny organisms made of genetic material called nucleic acid—either DNA or RNA—that is enclosed within a protein capsule, 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 regular living cells in your body and use them to multiply, overtaking other cells, and continuing to reproduce. This process can damage or kill the regular cells, leading to illness.
While a virus is incapable of reproduction unless it's within a cell of another organism, bacteria—larger, single-celled organisms—are capable of living in various types of environments and reproducing themselves, says Dr. Bailey.
The human body is actually full of bacteria—some are harmless, and some are even helpful (like by helping to keep your gut healthy), 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. 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 Centers for Disease Control and Prevention (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.
How viruses and bacteria spread
First, let's break down how the germs are spread. Depending on the type, 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)
- Semen or vaginal discharge
That means measures like practicing safe sex, cleaning human-handled food like fruit and vegetables, and getting vaccinated against vaccine-preventable viruses can decrease your risk of getting infected by a virus.
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
Protecting yourself against infection-causing bacteria means taking steps similar to those for viruses, like treating water so it's safe for consumption, practicing safe sex, and vaccinating yourself and animals.
And of course, personal hygiene is key for protection against both bacteria and viruses. That's because general protective measures help to prevent many types of infections caused by bacteria (aka, bacterial infections) and viruses (aka, viral infections).
The hygiene basics are super effective: Wash your hands often, for 20 seconds, with soap and water. If soap and water isn't available, use an alcohol-based hand sanitizer. Regular hand washing helps prevent illness because viruses and bacteria can live on your hands. It's good practice to avoid touching your eyes, nose, and mouth unless it's completely necessary, and especially so if you've not washed your hands in a while.
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 it comes to an infection that can be transferred through direct contact, air, or droplets, the CDC recommends keeping your distance from people who are sick to reduce your chances of catching their infection.
And again, 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. After all, as Dr. Fioritio points out, both viruses and bacteria can be deadly.
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.
Common bacterial infections include strep throat, tuberculosis, and urinary tract infections. Common viral infections include the common cold, chickenpox, and genital herpes. 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 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 staph 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.
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.
"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.
If you are feeling ill and think you might have an infection, you can go to the doctors 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
- 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.