What type of organism cannot be classified as a true cell and is often considered a living entity?

A virus is often regarded as a living entity despite not being classified as a true cell. Unlike organisms such as protozoa, fungi, and bacteria, which are made up of cells that can replicate independently and carry out various metabolic processes, viruses lack the cellular structure and the machinery necessary for metabolism and reproduction. Instead, viruses are composed of genetic material (either DNA or RNA) encased in a protein coat. They can only reproduce by infecting a host cell and hijacking the host's cellular machinery to replicate themselves.

This unique dependency on host cells for reproduction and metabolic processes is what differentiates viruses from other organisms that are classified as true cells. As a result, while viruses exhibit some characteristics of living organisms, they do not meet all the criteria necessary to be classified as cells, making them a fascinating topic in microbiology and virology.