Slavery sustained sugar production under harsh conditions, including the unhealthy climate of the Caribbean, where diseases such as malaria (brought from Africa) and yellow fever caused high mortality. In 1787 alone, the French imported about 20,000 slaves from Africa into Saint-Domingue, while the British imported about 38,000 slaves total to all of their Caribbean colonies. The death rate from yellow fever was such that at least 50% of the slaves from Africa died within a year of arriving, so the white planters preferred to work their slaves as hard as possible while providing with them with the barest minimum of food and shelter. They calculated that it was better to get the most work out of their slaves with the lowest expense possible, since they were probably going to die of yellow fever anyway. The death rate was so high that polyandry—one woman being married to several men at the same time—developed as a common form of marriage among the slaves. As slaves had no legal rights, rape by planters, their unmarried sons, or overseers was a common occurrence on the plantations.