We're all familiar with computer viruses: bits of code which attempt to transmit themselves to another system, from which they look for yet more systems to infect. The key point is that the virus must transmit code to infect another system.
Very occasionally there are more obscure examples of viral behavior, of data alone unaccompanied by code. Certainly, there is code involved, but it is the normal code of the system which performs a useful function and is not intended to spread data virally.
For example: people usually refer to me by the nickname Denny. My personal email address is denny@geekhold.com (1) but my work address is not "denny". One person had my personal email address. Shortly after I started he sent an email to a few team members. He typed "Denny," and GMail helpfully auto-completed my personal account from his contacts. When another member of the team responded to that email, the address was automatically added to his contacts. When he later sent an email to another set of coworkers... the Contact virus spread further.
The trouble with data-driven viral behavior is that there are few tools to stamp it out. With code viruses, a huge ecosystem of tools and malware signatures has been created. There are few tools to deal with an annoying bit of data spreading through the system, just manual exhortations to not respond to the email containing the external address.
Do people know of other instances of a viral spread of data, unaccompanied by code? There are certainly instances of poor auto-complete behavior, such as a mistyped URL leading to the browser forevermore suggesting the bogus site, but none I can think of which spread from one person to another. I suspect the root cause is a poor model for how auto-complete is supposed to work: the developer wants it to be completely automatic and not something the user should have to adjust.
(1): I long ago gave up keeping my email address off of spam lists. As I've used this address since 1996, I am wholly reliant on good filtering.