Viruses work in a variety of insidious ways.
Melissa, the fastest spreading virus ever when it debuted on March 26th, was sent out by e-mail. The virus can wreak havoc in many other ways and can change system security codes, potentially subjecting systems to future viral invasions. The laws make it a crime knowingly or intentionally to transmit damaging materials to a computer. So an individual user could be a criminal for spreading a virus they did not create. However, likelihood of your being prosecuted for secondary transmission is slight.