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What's the difference between Bacteria and Viruses?

Medically reviewed by Carmen Pope, BPharm. Last updated on April 3, 2023.

Official answer

  • Bacteria and viruses differ in their structure and their response to medications.
  • Bacteria are single-celled, living organisms. They have a cell wall and all the components necessary to survive and reproduce, although some may derive energy from other sources.
  • Viruses are not considered to be “living” because they require a host cell to survive long-term, for energy, and to reproduce. Viruses consist of only one piece of genetic material and a protein shell called a capsid. They survive and reproduce by “hijacking” a host cell, and using its ribosomes to make new viral proteins.
  • Less than 1% of bacteria cause disease. Most are beneficial for our good health and the health of Earth’s ecosystems. Most viruses cause disease.
  • Antibiotics may be used to treat some bacterial infections, but they do not work against viruses. Some severe bacterial infections may be prevented by vaccination.
  • Vaccination is the primary way to prevent viral infections; however, antivirals have been engineered that can treat some viral infections, such as Hepatitis C or HIV. Antivirals are not effective against bacteria.

What are bacteria?

Bacteria are simple, single celled organisms, called prokaryotes, which means their DNA is contained within a certain area of the cell called the nucleoid, but not enclosed. Bacteria are one of the oldest living things on earth, having been in existence for at least 3.5 billion years. A microscope is needed to see them.

Bacteria come in many shapes and sizes, including spheres, cylinders, threads, rods, or chains. They can be aerobic (those that require oxygen to survive), anaerobic (those that die when exposed to oxygen), and those that prefer oxygen but can live without it. Bacteria that create their energy through light or chemical reactions are called autotrophs, and those that have to consume and break down complex organic compounds to obtain energy are called heterotrophs.

Bacteria are enclosed by a rigid cell wall, which can vary widely in its composition, helping to distinguish between different species of bacteria. When exposed to a dye called a gram stain, gram positive bacteria trap the dye due to the structure of their walls, while gram negative bacteria release the dye readily, because their cell wall is thin. Inside the cell wall sits all the components necessary for bacteria to grow, metabolize, and reproduce.

Bacteria may also have protrusions, these are known as pili (help bacteria to attach to certain structures, such as teeth or intestines) or flagella (which help bacteria to move).

Although some bacteria can cause disease, less than one percent make us sick. Many beneficial species are essential for our good health and the overall health of most of Earth’s ecosystems. Inside our bodies, we have tens of trillions of bacteria making up our gut microbiome, and trillions more living, usually harmlessly, on our skin. Many chronic diseases, such as cancer and heart disease, are associated with poor oral health often because of an imbalance of bacteria within our mouth. Infections caused by bacteria include strep throat, tuberculosis, and urinary tract infections (UTI).

The primary way to prevent bacterial infections is by giving antibiotics; however, because of resistance, antibiotics are usually only used for severe infections, because the immune system of most people is usually strong enough to overcome the infection.

For some severe bacterial infections, such as diphtheria, meningococcal disease, pertussis, or tetanus, vaccinations have been developed and these are the most effective way to prevent against infection.

What are viruses?

Viruses consist of a piece of genetic material, such as DNA or RNA (but not both) surrounded by a protein shell called a capsid.

Sometimes this shell is surrounded by an envelope of fat and protein molecules, and out of this envelope may project glycoprotein protrusions, called peplomers, which can be triangular, spiked, or shaped like a mushroom. These protrusions bind only to certain receptors on a host cell and determine what type of hosts or host cell a virus will infect and how infectious that virus will be.

A microscope is required to see viruses and they are 10 to 100 times smaller than the smallest bacteria.

Because viruses MUST infect a host cell to carry out life-sustaining functions or to reproduce, they are not considered living organisms, although some can survive on surfaces for long periods. Viruses are essentially like a parasite, relying on a host cell to reproduce and survive.

When a virus infects a host cell, it uses its genetic material to “hijack” the ribosomes in the host cell. These are the cell structures that make protein. So instead of protein being made that can be used by the host cell, viral proteins are made.

The virus also takes advantage of other components within the host cell, such as ATP (adenosine triphosphate) for energy, and amino acids and fats to make new capsids and assemble new viruses. Once enough new viruses have been made, they burst out of the cell in a process called lysis, which kills the host cell. This is called viral replication and it is the way viruses reproduce.

Once new viruses have been made, they can go on to infect new host cells, and new hosts.

Most viruses cause disease, and they are usually quite specific about the area of the body that they attack, for example, the liver, the respiratory tract, or the blood. Common viruses include herpes zoster, HIV, influenza, the common cold, and the rabies virus. Viruses can also cause pneumonia or sinusitis. The new coronavirus SARs-CoV-2 that causes COVID-19 is also a virus.

As well as humans and animals, viruses can also infect plants, although virtually all plant viruses are transmitted by insects or other organisms that feed on plant walls.

The primary way to prevent viral infections is by vaccination; however, antivirals have been engineered that can treat some viral infections, such as Hepatitis C or HIV. Antibiotics do not treat a viral infection.

Virus vs bacteria: Any difference in symptoms?

Symptoms usually reflect the area of the body infected, and the infecting organism. For example, a bacterial infection of the skin may cause a discharge, swelling, pain and redness in a certain area, whereas a viral infection, such as hepatitis C may cause abdominal pain, joint pain, nausea or vomiting, and yellowing of the skin or eyes.

Some illnesses can be caused by either a virus or bacteria, for example pneumonia, meningitis, or diarrhea, and symptoms may be similar, reflecting the body trying to rid itself of the infecting organism, and may include:

  • Coughing
  • Cramping
  • Diarrhea
  • Nausea or vomiting
  • Sneezing
  • Tiredness.

Virus vs bacteria: Any difference in transmission?

Bacteria and viruses can be spread in similar ways, such as:

  • Being exposed to droplets expelled when a person coughs or sneezes in your vicinity
  • Close contact with an infectious person
  • Contact with infected surfaces and then touching your nose, mouth, or eyes
  • Contact with infected body fluids through kissing, sex, urine, or feces
  • Contact with infected animals or insects such as fleas, ticks, or mosquitoes
  • Transmission from mother to child during birth.

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