A medical scheme can ask about your medical history (open-enrolment doesn't mean no questions), but by law it cannot refuse to enrol you because you have a pre-existing condition. What it can do: apply waiting periods, apply a late-joiner penalty if you're over 35 with no prior cover, and place limits on condition-specific cover for the first 12 months.
SA example: You have HIV and apply to Discovery. They can't refuse you. They will apply a 3-month general waiting period and a 12-month condition-specific waiting period for HIV, after which full PMB-level cover kicks in for life.