Here's why:
* Lack of cellular structure: Viruses are not made up of cells. They are simply genetic material (DNA or RNA) encased in a protein coat. They lack the complex cellular machinery found in living organisms.
* Incapable of independent metabolism: Viruses cannot produce energy or carry out metabolic processes on their own. They rely entirely on a host cell to replicate.
* Not capable of independent reproduction: Viruses can only replicate inside a living cell. They lack the machinery for independent reproduction.
The cell theory states that all living organisms are composed of cells and that all cells arise from pre-existing cells. Viruses do not fit this definition, as they do not have cells and cannot reproduce without a host.
Therefore, the existence of viruses does not invalidate the cell theory. They are considered to be a unique type of biological entity that exists at the edge of life.
