Why did one-third of slaves die within three years on a plantation?

Only about 6 percent of the slaves imported in Africa ended up in what is now the United States. The overwhelming majority went to Brazil or the West Indies, where they labored on plantations producing sugar, tobacco, coffee, and other subtropic products. Of the 10 to 15 million Africans who survived the voyage to the New World, over one-third landed in Brazil and between 60 and 70 percent ended up in Brazil or the sugar colonies of the Caribbean. Yet by 1860, approximately two-thirds of all New World slaves lived in the American South. They work too hard and they push their selves so they wouldn't get hurt by their owners.

They worked for long hours, had little food, and could have been fighting off a disease