It also states that cells arise from other preexisting cells and this means that living cells can replicate and reproduce to form other cells. Now, the focus of this lecture will be viruses. And viruses are basically small agents that infect other cells and this includes animal cells, plant cells as well as all different types of other cells. Now, viruses do not actually satisfy the cell theory. For example, as we'll see in just a moment viruses cannot actually replicate and reproduce on their own. And that means viruses are not considered living organisms.