Phishers seem to be increasingly using social engineering techniques to infect victims. The most common case is posting a link in a social networking site or forum which points to an "interesting resource". Users click this link and land on a web page which claims that they need to download a codec or something similar, before they can access the "interesting resource". Once a gullible user downloads and executes the supposed codec, the trojan infects the user's system and makes it part of a botnet or something similar.
The video below is a demo from
Webroot Threat Blog about such an attack on gaming forums.