Once again we have an example of the dichotomy of control, with a practical twist.
Disease is an impediment to the body, but not to the will, unless the will itself choses. Lameness is an impediment to the leg, but not to the will. And add this reflection on the occasion of everything that happens; for you will find it an impediment to something else, but not to yourself.
In part 1, "body" is listed as one of the things which we do not control. Disease, lameness, or anything else that happens to the body is out of our control. For the most part though (with the exception of diseases that affect cognitive ability), things that happen to our body do not effect our ability to master our thoughts and actions.
It's easy to give yourself a pass when you're feeling ill; to put aside self improvememnt for time. Instead though, as we'll see in part 10, we should look on this as an opportunity to really put our Stoic logic into play.
The last line expands the concept from illness to everything in life: no matter what happens, nothing can stop you from being in control of your own mind.