WordPress get injected by malicious script

Hi Guy, my wordpress is injected by malicious script. I try to overwrite the index.php file but i just work for a while only then the script will be get injected again. May i know how to resolve the issue? is it related to the web host?? it was injected as index.php page and other page as well. the malicious script is as below:

[moderated – please don’t post exploit script here, it can trigger other people’s AV scanner]

Is there anyway i can remove it permanently??