Why are viruses considered microbes?
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.


There is no universally accepted definition of life.
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.