wordpress websites hacked

Hi,

All of the wordpress websites on my server have been hacked roughly once a month for the past 2 months now. Every time they are hacked, the same thing is happening. The index.php/header.php files in both the themes and the wp installation directory are being edited and some javascript is being added to those files which is then displaying on the websites. Each time this happens, i roll back the database and code for all the websites to the night before the websites were hacked. This clears up the problem until they are hacked again in around a month or so. I have ensured that we are running the latest version of wordpress and the latest version of each and every plugin we use. At the time the files were changed, the server logs report no access to the website. I’m reall scratching my head at this one – has anyone else encountered this problem and if so how did you solve it?