For most of us, the word “virus” has a negative aspect to it. We
hear the word and think either of getting sick, of a nasty little
bug that can make us sick, or of a computer virus which cost us
tons of data and hardship at one time or another.  In fact, viruses
are not necessarily bad, and most life on earth relies at least in
part on them in one degree or another.  Some people even argue
that humanity itself fits the classic definition of a virus, infecting
the planet earth for better or worse! 

Classically, the definition of a virus is a “small particle that lives
as a parasite in plants, animals, and bacteria and consists of a
nucleic acid core within a protein sheath.” In some cases this
parasitic co-existence may actually benefit the host in one way
or another, an existence known as a “symbiotic” relationship. 
It’s common knowledge that most viruses have the ability to
replicate themselves without any outside assistance, simply by
finding a medium in which they can exist.  In other words a virus,
when introduced into a system, will make copies of itself.  Due
to their nature, most viruses are easily moved from one system
to another as they replicate, causing them to spread quickly
and easily from one host site to another.

When it comes to marketing and viral marketing campaigns, the
idea is much simpler and focuses on the fact that most viruses
(real or computerized) have the ability to replicate themselves
without “outside” assistance.  In this case a “viral” marketing
campaign is one which allows or stimulates customers who are
exposed to the “viral” marketing campaign to actively help
spread the word themselves.

That is, they send your promotion piece on to other potential
customers due to their excitement with the product it
underscores or because of they found it entertaining as a result
of containing something humorous, or worth sharing as the result
of a gift or award option associated with the campaign. 

Having such a built-in replication capability, just as with real
viruses, it has the potential for exponential growth and can
quickly spread a message to thousands or even millions of users
from a single tarting point.  Urban myths are a good example of
such growth. Even though numerous websites debunk such
myths, they continue to circulate widely. More than half the
people exposed to them forward them to others even though
they’ve heard them already and know they are false. Sooner or
later, the myths have spread too far and wide to stop. 

One of the greatest examples of this in an actual working
scenario was the original hotmail expansion program. Hotmail.com,
as one of the first “free” web e-mail service group, began
recruiting members by first giving away free e-mail addresses and
services, then requiring that every e-mail sent via their service
include a simple tagline at the bottom detailing that the receiver
could also receive free e-mail by signing up at hotmail.com.

Like ripples from a rock flung into a pond, the initial offering
quickly spread outward causing exponential growth and a huge
list of members to be tapped for marketing other services
towards.

So the primary way Viral Marketing us unique is, it has the
potential to self promote by word of mouth exponentially.