Simplify the following text for a Grade 11 native English-speaking person.
Original: The cutthroat trout has also faced an ongoing drought , as well as the accidental introduction of a parasiteâ '' whirling diseaseâ '' which causes a terminal nervous system disease in younger fish .
Simplified: The cutthroat trout has also faced an ongoing drought , as well as the accidental introduction of a parasiteâ '' whirling diseaseâ '' which kills younger fish .