Viruses vs Bacteria - A Comparison of Infectious Microorganisms

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What is a Virus?

A virus is a microorganism that cannot grow or reproduce without a host cell. It isn’t a cell, and it isn’t considered alive. It invades a host cell and causes disease. Nearly all viruses are harmful to the host. They are usually treated with antiviral medications and prevented with vaccinations. The seasonal flu and the common cold are caused by viruses. Scientists have found beneficial uses for non-pathogenic viruses in a laboratory setting through genetic engineering.

What are Bacteria?

A bacterium is a microorganism consisting of a single cell. Some types of bacteria live apart from other organisms, while other types of bacteria are parasitic and live within a host organism. Some types of bacteria are helpful to the host, such as the bacteria normally present in the gastrointestinal tract. Other types of bacteria are harmful to the host and cause disease. Antibiotics are used to treat bacterial infections, such as strep throat and pneumonia.

Difference in Size

One of the differences that is noticeable in a comparison of viruses and bacteria is the size of the microorganism. Viruses are much smaller than bacteria. A typical virus is between 20 and 400 nanometers, while a bacterium can be as large as 1000 nanometers. The smallest bacteria are usually visible with a light microscope, while viruses require an electron microscope for observation.

Difference in Structure

A virus is composed of genetic material, either DNA or RNA, and protein. The genetic material is contained within a capsid, which is made of protein. The capsid protects the genetic material from the surrounding environment. This is different from bacteria, which features free floating DNA and RNA in the cell’s cytoplasm. Bacteria also contain a cell wall and cell membrane.

Difference in Reproduction

The main difference in a comparison of viruses and bacteria is the way they reproduce. Viruses cannot replicate their genetic material or produce new viruses outside of a host cell. A virus lands on the surface of a host cell and deposits its genetic material into the cell. By using the host cell’s ribosomes and transcription enzymes, the virus produces proteins required for the structure of new viruses. Depending on the virus, it uses the host’s replication enzymes or its own viral enzymes to generate a copy of the genetic material. When the genetic material and proteins are produced, they assemble together in the host cell’s cytoplasm. Then, the newly created viruses exit the host cell and spread to other cells.

Bacteria replicates through a process called fission. It is a form of asexual reproduction where two cells, each containing a copy of the genetic material, are produced from one cell containing one copy of the genetic material. In the process, the genetic material is replicated and travels to opposite ends of the cell. The cell stretches and begins to enclose around each copy of the genetic material. Eventually, two separate cells are formed, divided by a cell plate.


1. “Definition of Virus.” MedicineNet.

2. “Definition of Bacteria.” MedicineNet.

3. “Bacteria vs Virus.” Diffen.