Do eye diseases affect refractive errors?

Sometimes: The focus of the eye is greatly affected by the lens - diseases such as cataract and presbyopia can dramatically affect the focus. Sometimes corneal scarring may induce astigmatism and keratoconous may cause irregular astigmatism requiring contact lenses to treat.

Yes: A myopic or hyperopic refractive shift can occur if there is anything that changes the shape of the cornea, affects the position of the lens (such as ciliary body issues), or changes the retina distance from the lens (i.e. Swelling).

Diabetes: There are also diseases that affect the entire body that can affect refractive error. Diabetes is the classic, and can cause huge changes, that usually reverse themselves when the blood sugar normallizes.