If you have .class element defined inside header.html then you need to use event delegation as below:
$(document).on('click','.class',function () {
        $('#id').slideToggle('fast');
});
Event delegation should be used on controls which are added to DOM at later part or after DOM gets loaded.
UPDATE
When you attach event directly to element like below:
$('.element').click(function(){
....
});
or as below:
$('.element').on('click',function(){
....
});
those event will not get attached to the dynamically added elements or elements which are added after DOM load. So you need to attach event either to document targeting particular element in DOM as below:
$(document).on('click','.element',function(){
    ...
});
or to its parent which was loaded during page load. For Ex:
Say you have below structure:
<div class="somediv">
   <!--Many other children elements here-->
</div>
Now you will add one more event either by append or by load or in any other possible ways and structure will become as below:
<div class="somediv">
  <!--Many other children elements here-->
  <button class="element">Click here<button> <!--This is dynamically added element -->
</div>
and now you can attach event to .somediv instead of attaching to document, since it was added during DOM load and newly added element will be inside this .somediv. 
This will actually improve performance of code as you are explicitly
  telling jquery from where to start to search for the dynamically
  added .element and jquery can search it more faster instead of
  traversing from document
So you can write it as below:
$('.somediv').on('click','.element',function(){
 //^^^^parent div         ^^^dynamically added element
    //do anything here
});