Simplify the following text for a Grade 12 native English-speaking person.
Original: Sickle-cell disease , usually presenting in childhood , occurs more commonly in people ( or their descendants ) from parts of tropical and sub-tropical regions where malaria is or was common .
Simplified: Sickle-cell disease occurs more commonly in people ( or their descendants ) from parts of tropical and sub-tropical regions where malaria is or was common .