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My question is simple. Why is a virus considered a microbe? Considering a microbe is considered to be a "living" unit of life, which viruses are not.

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    $\begingroup$ "Life" is either a) arbitrarily defined or b) poorly defined. $\endgroup$ – Nick T Oct 12 '15 at 7:23
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    $\begingroup$ Who considers a microbe to be "a living unit of life"? Do the people who use the word "microbe" use strict scientific definitions of life? In my experience, "microbe" is not a scientific term (although it may have been one once), it's a folklore word for "tiny bugs which make you sick". When a grandma tells her grandchild "cover your mouth when you sneeze, you're spreading microbes everywhere", she neither knows nor cares whether a biologist would classify the microbes as alive. Your question seems to need no biologic answer, but an epistemological one. $\endgroup$ – rumtscho Oct 12 '15 at 10:01
  • $\begingroup$ I think these sort of questions arise because we are generally too lazy to properly define terms. I've asked a similar question before: "Why isn't a virus alive"? We need to carefully define "life" every time it's brought up: Genetic code? Membranes? Ability to replicate? High cognitive power? Each has problems & caveats, but without a measurable feature, it's a moot discussion. I'd suggest reading The Seven Pillars of Life by Koshland, 2002. $\endgroup$ – James Aug 3 '16 at 3:44
  • $\begingroup$ Some recently identified viruses are considered alive due to their more complex organisation such as mimiviruses. Take a look at This it's pretty interesting. $\endgroup$ – Dart Feld Nov 9 '16 at 20:03
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What is a microbe?

A microbe (or microorganism) is a microscopic organism. Anything that is considered alive and that is small enough is called a microbe.

Note that this definition has two issues.

  1. There is no universally accepted definition of life.
  2. There is no universally accepted size threshold for being called a microbe (to my knowledge). I would go with a threshold of about $10^{-5}$ meters.

Is a virus a microbe?

A virus IS and IS NOT alive depending on the definition. Note btw, that the definition of what is alive is not a matter of Biology but a matter of Philosophy. Most of the time, viruses are considered as not being alive. It is important to understand that the definition of life has absolutely no impact on biology and is nothing but a question of nomenclature.

If you want to call a virus a living thing, then yes, viruses are microbes. As stated on the wiki article:

Some microbiologists also classify viruses (and viroids) as microorganisms, but others consider these as nonliving.

You can find a discussion of why viruses are generally considered as not being alive here.

Unit of life

The concept of "unit of life" has not much meaning in biology and, to my experience, is most often used as a nice image for teaching young students what a cell is. Most of the time that I heard of "unit of life" (mostly when I was in secondary school and eventually high school) was used to describe a single cell making up a multicellular individual such as a cell of your blood for example and not a unicellular individual.

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Viruses are not alive until they are inside the body of the host organism (plant or animal). Viruses are active once they are inside the host and reproduce inside the host. They are a link between living and non-living.

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Viruses are small bits of genetic code in a protective covering. Viruses are not "alive," that is, they cannot replicate, unless they are inside another organism. A virus is definitely too small to be seen without a microscope. Since viruses are so small (tinier than bacteria) they may be considered microbes. However, since they are not "alive" outside of a host organism, it is debatable whether they are really organisms at all. For convenience since they are neglected in other areas of biology, viruses are discussed in microbiology. Most viruses are known because they cause disease.

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    $\begingroup$ The claim [some organisms] are not "alive," that is, they cannot replicate, unless they are inside another organism is pretty bas IMO because following this logic, an endosymbiont cannot be a living thing. Plasmodium wouldn't be a living thing for example. Note also that stating Viruses are not alive reduces the discussion to a single definition while several exist (it has the advantage to highlight the result most common definition though) $\endgroup$ – Remi.b Oct 12 '15 at 8:16
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    $\begingroup$ @Remi.b: My personal definition, as a theoretical ecologist, is that if you can meaningfully study its ecology and evolution, then it's alive. :) That easily includes viruses. Of course, if you define "ecology" and "evolution" broadly enough, that definition can also cover a lot of things we would not usually consider alive, such as languages and ideas. I'm not really sure if that's an argument for or against it. $\endgroup$ – Ilmari Karonen Oct 12 '15 at 14:57
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    $\begingroup$ @IlmariKaronen As a theoretical evolutionary biologist, I like your definition! However, it is not the most common definition. In any case, I think pretty much everybody agrees that excluding from the realm of living things, everything which reproduction depend on other life forms is quite a bad idea! $\endgroup$ – Remi.b Oct 12 '15 at 18:56

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