Viruses are too small to be seen by the naked eye.
They can't multiply on their own, so they have to invade a 'host' cell and take over its machinery in order to be able to make more virus particles. They are capable of latching onto cells and getting inside them. The cells of the mucous membranes, such as those lining the respiratory passages that we breathe through, are particularly open to virus attacks because they are not covered by protective skin. Bacteria are organisms made up of just one cell and are too tiny to be seen by the naked eye. They are capable of multiplying all by themselves, as they have the power to divide.