In its simplest expression, a viral campaign is about making something people like and want to talk about.  The message has to be engaging, but implicitly give the sender some "kudos" in sharing it wider and really "like" the source of the message for creating it.

What it isn't is prosaic rubbish shoehorned into a fun message as some overt marketing campaign.  So key ingredients of a successful viral campaign?

a) Really understanding your audience;

b) Appeal to basic emotions cleverly, not crassly. Slapstick and "boobs" are out; tunes, romance, humour, pique are in.

c) Introduce an element that will encourage people to share it - uniqueness, querky, references celebrity - without upsetting what is good about the product itself, 

d) Don't re-use other peoples' ideas (yawn!) - or if you do - add a whole new twist to it;

e) Assume people won't be interested;

f) Leverage existing relationships;

g) Never SPAM