Viruses are considered to be non-living entities because they lack the characteristics of living organisms, such as the ability to reproduce on their own, grow, or metabolize. Instead, they rely on host cells to replicate.

All other living organisms, from the smallest bacteria to the largest whales, are made up of one or more cells. This is the fundamental basis of cell theory, which states that:

* All living things are composed of cells.

* Cells are the basic unit of life.

* All cells arise from pre-existing cells.