JavaScript setTimeout() won't wait to Execute? [duplicate]

Consider the following example:

<script type="text/javascript">
    function alertBox(){
        alert('Hello World!');
    function doSomething(){
        setInterval(alertBox(), 5000); //This is for generic purposes only
    function myFunction(){


What is it that causes this to execute IMMEDIATELY, rather than waiting the 3 seconds set, as well as only executing the alert ONCE, rather than at the scheduled 5 second intervals?

Thanks for any help you can provide!