Lawyer, should start your own topic. But to answer your question quickly it depends on the type of webserver you're using. It's more than likely that the server is getting compromised then getting infected from an outside source. I'd make sure to change all your passwords and if it happens again send an e-mail to your webserver indicating that there is a security breach on the server and that it should be fixed.
If all else fails switch hosting companies, there are thousands out there that will provide you with better service.
