Back end messed up/Virus!

Does anyone know what is going on with this admin panel issue that so many people seem to be having? My admin panel appears for a second then goes blank and firefox says ‘stopped’.

Avast keeps showing trojan warning saying ‘Sign of “JS:downloader-AI [Trj]” has been found in “https://news.hermison.com/” file.

I have no idea what news.hermison.com is.

Another thing I have noticed is that all my php files seems to have about 20 lines of gibberish beginning with <?php /**/ eval(base64_decode