Viruses come in different forms.
Some are mere nuisances some come with devastating consequences.
E-mail worms are self replicating and clog the networks with unwanted traffic.
Virus codes are not necessarily complex.
It is necessary to scan the systems/ networks for infections on a periodic basis for protection against viruses.
Antidotes to new virus releases are promptly made available by security companies and this forms the major counter measure. [1]
[1] (Acknowledgement: http://www.pchell.com )