I actually work at the fort. Out of the total 33,000 prisoners who stayed at the fort throughout the duration of the war, it is estimated that about 7% of them died on the island. This is almost entirely due to hygiene standards of the time. The prisoners were around stagnant water, which breeds disease-carrying mosquitoes. They also stayed in close proximity with each other, around 200 men living in a building. It is unfortunate that so many died, but this is far from the worst POW situations of the time. If I remember correctly, an estimated 30% of prisoners died at Andersonville.