Explanation:
viruses are very small -- 100 times smaller than the average bacterium, so small that they can't be seen with an ordinary microscope. Viruses can only exert influence by invading a cell, because they're not cellular structures. They lack the ability to replicate on their own, so viruses are merely tiny packets of DNA or RNA genes enfolded in a protein coating, on the hunt for a cell they can dominate.
