WordPress › Support » <iframe> virus - .cn1) First of all, it is not a wordpress problem... it is your problem.
2) The attack is caused by a trojan that resides in your computer, 99% that you have windows.
3) This trojan stoles all your FTP credentials (and who knows what else), and are used to access your site accounts and modify your code, inserting an <iframe> in some cases. In other cases, these kind of virus insert a compressed javascript code.