A very subtle sort indeed, traffic viruses are adverts —links actually— whose action automatically generates additional traffic.

You have probably wondered why so many sites offered free electronic postcards of all kinds. Suppose you send one to a friend. He or she does not receive it directly. They have to go retrieve it from the site, don't they? So this ensures traffic. But as a rule, the newcomer on the site will find in this attractive environment good reasons for sending someone a card as well. This translates into even more traffic, and so forth.

In a Metacrawler results page —quite often actually on engine results pages— you get this interesting line that offers to "email results to a friend". This is a very useful device for us users. You think by clicking you will be sending this result page to someone interested, e.g., your students. Good information for them to chew on. But what happens is that your recipients will be given in their e-mail a URL to click on, ie, Metacrawler's address combined with the request that produced these results. So that basically, as a teacher, you have generated traffic on Metacrawler's site. It is not that bad, but it is important to be aware of it. Especially as this new traffic on the site entails of course more profit for Metacrawler, who charges advertising in direct proportion to the number of users.

Quite frequently as well on engine results pages you get this other interesting line that says "Find books on blahblah at bn.com, or amazon.com. 'bn' stands for Barnes and Nobles, a well-known online bookstore, like Amazon. What they do is actually take your request and hand it over to the engine that searches the bookstore's database, which prepares a shopping list for you. This is of great use for us teachers.
For example I was once looking for Blade Runner -related websites. Out of curiosity, I clicked on that link which proposed books on Blade Runner at bn.com. After I had selected one from the list, the Barnes and Nobles search robot offered a whole list of related books which typically had been bought by user-clients along with the one I chose. This was a bibliography, and a very good one at that, as I finally opted for another book, far better than the one I had previously selected. This was a mind-opener of the best kind.
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That said, clearly with this very small prompter Barnes and Nobles generates new traffic, increasing the number of potential buyers on their website, while Metacrawler increases profits by receiving commission from B&N as a result of my buying spree. And so forth...