Some viruses spread by E-mail
Melissa was the first other notable examples include my personal favourite "I love you".
Victims would get a mail from someone they knew with the subject line "I love you"
They'd open it ( well you would wouldn't you ?) which would activate the virus which would then send a copy of itself to everybody in the victim's address book.
This was 10 years ago this sort of virus is rarer these days, nowadays it's more likely to be password stealing trojans or websites that tell you you have a virus and try to get you to pay money to remove the non-existant infection, or worse infections that they download themselves