Doing anything over and over again can get pretty annoying. Think of bursitis as your body getting annoyed with you. Tiny fluid-filled sacs, called bursae, cushion your tendons from your bones, making movements around your knees, ankles and other areas smooth. Heavy, repetitive motion like running can irritate the bursa and can cause inflammation and pain.
Bursitis is characterized by a general pain or burning surrounding the bursae area. In the knees, this area is immediately above or below the patella, or kneecap, and behind the tendons as they approach the patella.
What you don't want to do is force your way through it. If the inflammation becomes too bad, the bursae can break open and cause infection, which in severe cases may even require surgery to repair.
The best treatment for bursitis is to remove the pressure on the bursa. Cortisone injections can cause more harm than good and should be avoided. For most people, the pain will subside in a few weeks. In more serious cases, you should consult a good sports doctor for an expert opinion.
You're better off not needing any of the above advice and avoiding bursitis completely. To minimize your chances of falling prey to this problem, make sure you warm up properly before running, and stretch your legs well. This will loosen up your tendons, thereby reducing the friction on the bursae [source: Schwartzburg]. Your knees will thank you for it.