Bacteria vs Virus

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Bacteria are single-celled, prokaryotic microorganisms that exist in abundance in both living hosts and in all areas of the planet (e.g., soil, water). By their nature, they can be either "good" (beneficial) or "bad" (harmful) for the health of plants, humans, and other animals that come into contact with them. A virus is acellular (has no cell structure) and requires a living host to survive; it causes illness in its host, which causes an immune response. Bacteria are alive, while scientists are not yet sure if viruses are living or nonliving; in general, they are considered to be nonliving.

Infections caused by harmful bacteria can almost always be cured with antibiotics. While some viruses can be vaccinated against, most, such as HIV and the viruses which cause the common cold, are incurable, even if their symptoms can be treated, meaning the living host must have a strong enough immune system to survive the infection.

Comparison chart

Bacteria versus Virus comparison chart
	Bacteria	Virus
Introduction (from Wikipedia)	Bacteria constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals.	A virus is a small infectious agent that replicates only inside the living cells of other organisms.
Ribosomes	Present	Absent
Cell wall	Peptidoglycan / Lipopolysaccharide	No cell wall. Protein coat present instead.
Living attributes	Living organism	Opinions differ on whether viruses are a form of life or organic structures that interact with living organisms.
Nucleus	No	No
Number of cells	Unicellular; one cell	No cells; not living
Structures	DNA and RNA floating freely in cytoplasm. Has cell wall and cell membrane.	DNA or RNA enclosed inside a coat of protein.
Reproduction	Fission- a form of asexual reproduction	Invades a host cell and takes over the cell causing it to make copies of the viral DNA/RNA. Destroys the host cell releasing new viruses.
Treatment	Antibiotics	Vaccines prevent the spread and antiviral medications help to slow reproduction but can not stop it completely.
Enzymes	Yes	Yes, in some
Virulence	Yes	Yes
Infection	Localized	Systemic
Benefits	Some bacteria are beneficial (e.g. certain bacteria are required in the gut)	Viruses are not beneficial. However, a particular virus may be able to destroy brain tumors (see references). Viruses can be useful in genetic engineering.
Size	Larger (1000nm)	Smaller (20 - 400nm)

Virus - Bacteria Differences

Scanning electron micrograph of Escherichia coli bacilli

Viruses are the smallest and simplest life form known. They are 10 to 100 times smaller than bacteria.
The biggest difference between viruses and bacteria is that viruses must have a living host - like a plant or animal - to multiply, while most bacteria can grow on non-living surfaces.
Bacteria are intercellular organisms (i.e. they live in-between cells); whereas viruses are intracellular organisms (they infiltrate the host cell and live inside the cell). They change the host cell's genetic material from its normal function to producing the virus itself.
There are some useful bacteria but all viruses are harmful.
Antibiotics cannot kill viruses, but can kill most bacteria, with the exception of most Gram-negative bacteria.
An example of a disease caused by bacteria is strep throat and an example of an affliction caused by a virus is the flu.

Video Explaining the Differences

This video explains the overall differences between bacteria and viruses.

Structure and contents of a typical Gram positive bacterial cell

Differences in Reproduction

Bacteria carry all the "machinery" (cell organelles) needed for their growth and multiplication. Bacteria usually reproduce asexually. In case of sexual reproduction, certain plasmids genetic material can be passed between bacteria. On the other hand, viruses mainly carry information - for example, DNA or RNA, packaged in a protein and/or membranous coat. Viruses harness the host cell's machinery to reproduce. Their legs attach onto the surface of the cell, then the genetic material contained inside the head of the virus is injected into the cell. This genetic material can either use the cell's machinery to produce its own proteins and/or virus bits, or it can be integrated into the cell's DNA/RNA and then translated later. When enough "baby" viruses are produced the cell bursts, releasing the new viral particles. In a sense, viruses are not truly "living", but are essentially information (DNA or RNA) that float around until they encounter a suitable living host.

Transmission electron microscope (TEM) image of a recreated 1918 influenza virus

Abundance

Viruses are ten times as abundant as prokaryotes like bacteria. Hundreds of millions of viruses can be found in one square meter; the same space holds tens of millions of bacteria. In her book Viruses: A Very Short Introduction, Dorothy Crawford writes:

There are around 1 million different viral species in a kilogram of marine sediment where they infect and kill co-resident bacteria. Overall, marine viruses kill an estimated 20-40% of all marine bacteria every day, and as the major killer of marine microbes, they profoundly affect the carbon cycle by the so-called 'viral shunt'.^[1]

An article in Nature also confirms that viruses outnumber prokaryotes by ten to one and kill half of the world's bacteria every two days.

Given that bacteria are able to grow and reproduce at an exponential rate — only constrained by nutrients in the environment — viruses contribute to maintaining a healthy balance in the ecosystem.

Living vs. Nonliving

Bacteria are living organisms but opinions vary on whether viruses are. A virus is an organic structures that interacts with living organisms.

It does show characteristics of life such as having genes, evolving by natural selection and reproducing by creating multiple copies of themselves through self-assembly. But viruses don't have a cellular structure or their own metabolism; they need a host cell to reproduce. Viruses inject their own DNA into the host; sometimes those new genes are useful to the host and become part of its genome. It is estimated that up to 8% of our genome actually consists of endogenous retrovirus DNA.

It should be noted that bacterial species such as rickettsia and chlamydia are considered living organisms despite the same limitation of not being able to reproduce without a host cell. See also Wikipedia's page on the life properties of viruses.