Final answer:
A virus is a non-living particle consisting of genetic material surrounded by a protein capsid. It lacks cell structures and must infect a host cell to replicate.
Step-by-step explanation:
A virus is not a cell, but rather a non-living particle consisting of genetic material surrounded by a protein capsid. It lacks cell membranes, organelles, and the ability to reproduce on its own.
Viruses are considered obligate intracellular parasites as they must infect a host cell to replicate. They hijack the host cell's machinery to produce viral components and form new virus particles.
Viruses exhibit various shapes and can infect a wide range of organisms including animals, plants, bacteria, fungi, and even archaea. Despite not being classified in the tree of life, viruses possess genetic material and can evolve.