This one has been around since 2007. While it hasn't received as much media attention as Conficker or Storm, it reportedly holds its place as the second largest spam botnet in the world, sending approximately 7.7 billion e-mails per day, according to MessageLabs.
But its lack of notoriety is no fluke. In fact cyber criminals have gone to great lengths to keep this one under the radar, according to a Trend Micro report. For one, Pushdo authors have masked different variants of the malware, many of which come up as "generic detections." In addition, almost all Pushdo components reside in memory, making detection even more difficult. Plus, the botnet does not contain any means of self replication, while its owners frequently update the botnet's code and functions.