@umbra21 - It actually can occur in healthy people as well, who aren't being stressed emotionally. We learned about it in a sports clinic.
If you push yourself too hard, even in a single, long session, you can cause leukocytosis in yourself. Not that you should be aiming to do that, as it's obviously a bad thing to do.
But, the body will start responding to you pushing that hard.
It's because of inflammation. If you work out too vigorously, you're going to cause a bunch of inflammation in your muscles and joints and your body interprets that as being sick.
So, the white blood cells are made to deal with it. In this case, generally it goes away by itself. It is a normal response, after all.