This is an arrangement between the student, professor, and Dean of Studies Office.  If a student is unable to make class due to their chronic health impairment, he or she will contact professors and the Dean of Studies to let them know that they are having a particularly bad day.  The student will then be excused.  However, a professor can decide that a student has too many absences to complete his or her course.  Thus, it is important for the student to arrange this accommodation with their professors at the beginning of the semester.