Hi Chris,
So the hack may have originated from your server then.
If you had access to your dashboard then you could have tried installing and running this anti-malware plugin (http://wordpress.org/plugins/gotmls/), but that doesn't sound possible in your case.
Your best bet is to follow the advice in the codex to remove the hack with password changes, clean install of wordpress core files, clean backup, etc:
http://codex.wordpress.org/FAQ_My_site_was_hacked.
And then when you're back up and running consider hardening your Wordpress installations and also adding the 5G firewall (which works well alongside Wordfence).
I would discuss all these steps with your host and make sure that they both agree with the process, and that they will be working at the server level to maintain/upgrade their security and ensure that the other infected sites are cleaned by the site owners or quarantined.
Barnez