If your server does not support separation at the domain level, i.e. within your hosting account, then other pages on the server may be infected as well.
After modifying files located on the server and running a script on it, the attacker gained access to all files located within your account. Therefore, it can snoop them, which means that access data are compromised, and the hacker (scripts) can modify them (i.e. backdoors can be anywhere on your account).
Choosing good hosting is crucial, especially if you want to host more than one website on one server. If you’re going to keep more than one website, always choose a hosting with the domain separation on a server level.