What part of the cell theory most implies that viruses for all of their similarities to living cells cannot be considered a life form according to definition

A cell theory states that all living beings are made up of cells, cells are the basic units of structure and function unit of living individuals, it suggests that all cells comes from other cells. Viruses violates this theory as viruses resembles living organisms but they actually cannot be considered as living because they are not made up of cells, instead a virus structure includes a protein coat surrounding a piece of DNA or RNA. Viruses can adapt to the environment and respond to the stimuli, but they do not use energy and they do not show growth. The cells requires energy and they also show growth.  

According to cell theory cell is a structural component of living organisms but virus structure does not agree with this theory. Hence, virus cannot be considered as a life form according to the cell theory.    

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