Usually anything "viral" would have to be bad, as they're what attack us and make us sick. We fight them with antibodies. But then, antibodies need to multiply exponentially and spread in clever ways (e.g. through the invention of vaccination, a way of spreading immunity) .
"Organic propagation" is often a good thing, both energy efficient and effective.
So "viral marketing" is sort of in between, as "marketing" sounds positive (we like shopping) and so "viral" sounds clever -- yet vaguely sinister. Kinda slimey and snake oily. Kwel.
I didn't invent this term, just one of my MySpace friends uses it in his profile, and I've been giving the matter some thought.
Basic memes are the same way. The idea of "an alphabet" apparently spread like wildfire, though not every culture chose that solution, let alone the same alphabet.
The fact of competing solutions needn't be interpreted as a threat to survival nor a weakness in humanity, but should be celebrated for the opportunities to collaborate it continues to assure us. Monoculture is dangerous and we deliberately scare ourselves with its prospect, in nightmarish science fiction (but on the other hand, we appreciate having shared criteria for what's "humane versus inhumane treatment" i.e. universal human rights).
This is one of the USA's great teachings: compete in the marketplace of ideas as civilians, and you'll prosper a lot more (i.e. don't take gun play too lightly, even if they do on TV). But that doesn't mean everyone who comes here on a boat or a jet ever learned this lesson. It's an ongoing challenge to civilization itself: how to model or simulate well enough to anticipate breakdowns. We're getting better at it.