You can't give yourself a tooth-ache and expect everyone who comes in contact with you to get one and further spread tooth-aches to others right?
If you want to spread a disease you have to think of catching an illness that's contagious.
When creating viral marketing, it is necessary to create something that is so interesting and infectious that people automatically share it with everyone they know.
What I have seen a few people do, is send it to you and insist you send to everyone on your mailing list.
Not only does that not help it also make you lose your credibility. People will start to ignore forwards from you.
Secondly, why do we treat viral ads as separate from mainstream TVCs? A truly entertaining/interesting TVC will automatically get spread on the web.
We must strive to make every ad viral!