It's most likely a vulnerability in a plugin installed on one of your domains. Your theme could even be using a vulnerable version of timthumb.php. Most reputable hosts scan for vulnerable versions of this file and notify you of it being updated.
Hostmonster.com makes you set a very length and strict password with both upper case lower case and special characters. Clear text passwords shouldn't be stored anyplace it's bad security for your host to be allowing that.
That said it's most like your WordPress sites which were vulnerable and now affected. You should download the entire file system from one WordPress site, re-install WordPress with a fresh copy. Do the same with all the plugins then compare files. You may find some extra .php files on your backup which could be backdoors into your system.
There are a few entries which could have been added to your wp_options table. At the least some base64 code in your index files. After determining that your core WordPress files and plugins have been updated to the latest versions and that no other PHP files are on your site including checking hidden directories and files beginning with periods (.) and making sure no new entries in your db were made. Then change all your passwords MySQL, FTP, Control panel, email, WP Logins etc. Then scan your local PC for any malware.
Install WP File monitor, it'll notify you next time any files are changed.