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.
Viruses consist of genetic materials (DNA or RNA) surrounded by a protective coat of protein. 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.
