In "real life", viruses transmit by attaching to a host cell, then replicating exponentially. Similarly, information online can be shared "virally" - if information is given to two people, and each receiver tells two or more people, the information has the potential to be incredibly far-reaching, especially given the global nature of the internet.
An example of information transmitted "virally" online is that of the Mumbai Bombings in November of 2008. It is estimated that witnesses to the attacks sent around 80 "tweets" every five seconds during the siege, which lasted three days (source). The subjects discussed ranged from actual accounts of the events ("Mumbai terrorists are asking hotel reception for rooms of American citizens and holding them hostage on the floor") to calls for aid, including the broadcast of emergency telephone numbers and locations for people to donate blood for victims.
For many people, the first news they heard of the Mumbai attacks was via Twitter. Since the service is free, mostly uncensored and incredibly far-reaching, new information - however biased and unconfirmed - was readily available online.
The Taj Mahal Palace and Tower, South Mumbai
Facebook allowed users to "check up on" friends and family who may have been affected by the attacks. My partner at the time was on holiday in Hoshiarpur (admittedly, nearly 2000km from Mumbai - but the fact that he was in India at all concerned many friends here) and was able to alay our fears by posting the following Facebook status update: "alive and well, far away from "Bomb-Bay" (haha, get it?)" which drew a mixed response from our friends - some who commented "too soon" on his rather offensive play on words, to some who were relieved and passed the message on - another form of viral communication - to other friends. He became the guy that was "almost in Mumbai during the attacks".
In the wake of the Twitter explosion following the siege, the Indian government released a statement requesting that live Twitter updates from Mumbai "to cease immediately. ALL LIVE UPDATES - PLEASE STOP TWEETING" via news websites. This is comparable to the after-effects of the Sichuan Earthquake in China, where the Chinese government "shut down the social media" used to inform and co-ordinate the protests that followed the quake itself (source).
The internet has fairly revolutionised the "word-of-mouth" method of information transmission, and with the constant advances in technology and social media, this is likely to continue.