Hi, Linda,
The files impacted were the root index.php and two files were added to system/modules/pages.
There’s no way to advise on how to prevent this without knowing exactly how the hackers got into your system. It is possible that they didn’t hack your site directly, but came in through a script on another user’s account.
The only way to know is to email your host and ask them to help how the hackers got in; this should also alert them to the security issue. If you get no help from them; then honestly? I’d recommend another host. EngineHosting comes to mind - and security is very important to them. =)