Malicious code in Twenty Eleven Version 1.5 Header

I noticed that the text on my blog was changed to italic type today. Looking at the displayed source code, I discovered some malicious code. Upon further investigation, I found the source in the Theme header. I updated the Theme on a different server and compared it. This code: <div id="page" class="hfeed"> in the header was changed to:

<div id='hideMe'> <p><i> Online Cash Advance Payday Loans <a href="https://www.link-url-removed.com/" title="Online Cash Advance Payday Loans">Online Cash Advance Payday Loans</a></div><script type='text/javascript'>if(document.getElementById('hideMe') != null){document.getElementById('hideMe').style.visibility = 'hidden';document.getElementById('hideMe').style.display = 'none';}</script><div id="page" class="hfeed">

Is anyone familiar with this malicious code? Any suggestions for how to protect my blog against this kind of malicious attack in the future?