Nineteen year old Ereck Plancher died after an off season conditioning session. His death was tied to carrying the sickle cell trait. He was aware of he was carrying it and was monitored regularly.
Nine athletes have died since 2000 because of complications relating to carrying the trait.
The condition does not prohibit athletes from competing but National Association of Athletic Trainers (NATA) had set forth a series of precautions for monitoring athletes with the trait.
ESPN - UCF player knew of sickle cell trait that caused death - College Football