Generally, grad school isn't that hard to get into unless you are talking about a very space-limited program. (For instance, my neice who went into an audiology master's/doctoral program limited to about eight students.) Unless you are in that kind of a competitive application pool, I'd leave it out of the equation. Even then, it might work against you if it appears you might not be able to fulfill the program due to your illness or that you are using it as an excuse. So much will depend on who is actually reading that letter, and unfortunately, not everyone is terribly enlightened.