Word Press posts polluted, how to prevent it ?

I have a simple word press website for a local fitness club http://ift.tt/1OKDUdv . I recently decided to check it out and see if anything is broken unfortunately its posts was hijacked with malicious script that inserted some link tags that was forwarding the user to difference sites. I have saved the scripts before removing them and here are they. http://ift.tt/1XFtUKi

1- I wan't to know this could happen ? 2- How can someone do it with XSS attacks ? 3- How to prevent it from happening again ? 4- Any wordpress plugins for security ?

via Chebli Mohamed