A key reason for the high death toll was the tidal wave of war and desolation that the slave trade unleashed into the heart of Africa. Huge numbers of people died being marched to the coasts of Africa from the interior as well as in an endless series of wars produced by the quest for new slaves. Millions more would die in concentration camps at both ends of the sea journey, and significant numbers would die due to the appalling conditions on the slave ships.
The financial profits of this slave trade helped build the economic foundations of America. It was not just the south. Northern business interests made huge profits too.
It is difficult to estimate the exact death toll that resulted from the transatlantic slave trade. There weren’t exactly people measuring these numbers at the time. What we looked at are historical estimates of how many people may have died in capture, during the voyage at sea, and due to disease, starvation and back breaking labor in the New World. But what is certain is that the slave trade was a genocide against the African people. The transatlantic slave trade was also the largest, long distance coerced movement of people in history.
The estimate of the number killed during the transatlantic slave trade varies anywhere between 6-150 million. The official UN estimate is 17 million (UN). However, we ourselves would be inclined to agree the figure of 60 million, given all the variables here, including the fact that during the entire period of the slave trade, Africa’s population did not increase. Some may argue that this is because Europe had advanced medicine and technology, while Africans didn’t. Yet during this era Asia wasn’t exactly at a sophisticated, technological level either. But their population nearly doubled. We believe the stagnation of Africa’s population is a byproduct of the transatlantic slave trade.
The transatlantic slave trade took place across the Atlantic Ocean from the 15th through the 19th century. The vast majority of those who were enslaved were deported to the New World, mainly on the triangular trade route and its Middle Passage. Most of those captured were West Africans. The numbers of Africans brought to the New World were so great, that they became the largest number of immigrants to be brought to the New World before the late 18th century. By 1820, nearly four Africans for every one European had crossed the Atlantic, and about four out of every five females that traversed the Atlantic were from Africa.
This was not the first time in history that Africans were kidnapped from their homes and enslaved. Europeans and Muslims engaged in the slave trade for centuries before the colonization of the America’s. Yet the transatlantic slave trade further bled Africa of its people and resources. While it was not the only slave trade in Africa, it was the largest in terms of sheer volume and intensity.
As Elikia M’bokolo wrote in Le Monde diplomatique:
The African continent was bled of its human resources via all possible routes. Across the Sahara, through the Red Sea, from the Indian Ocean ports and across the Atlantic. At least ten centuries of slavery for the benefit of the Muslim countries (from the ninth to the nineteenth)…. Four million enslaved people exported via the Red Sea, another four million through the Swahili ports of the Indian Ocean, perhaps as many as nine million along the trans-Saharan caravan route, and eleven to twenty million (depending on the author) across the Atlantic Ocean. 
The number of lives lost in the procurement of slaves remains a mystery, but according to the author of American Holocaust, it is likely that the number of slaves who died in procurement is equal to the number who survived. A database compiled in the 1990’s put the figure for the transatlantic slave trade at more than 11 million people. For a long time an accepted figure was 15 million, although this has recently been revised downward. Patrick Manning, the author of “The Slave Trade,” estimates that about 12 million slaves entered the Atlantic trade between the 16th and 19th century, and about 1.5 million died on board ships (others have put the estimate of ship deaths at 2.2 million), 4 million died inside Africa after capture and many more died young. These estimates also do not cover how many slaves died in the New World. 
THE JOURNEY ACROSS THE OCEAN
Aboard the ships, African captives were packed into tight, unsanitary spaces for months at a time. Many slaves who tried to starve themselves to death were force fed. These conditions also resulted in the spread of fatal diseases. Other fatalities were suicide, and slaves who escaped their fate by throwing themselves overboard. The slave traders would attempt to fit anywhere from 350-600 slaves on one ship. The journey typically took anywhere between 2-4 months, and during this time enslaved people were chained naked in rows on the floor of the hold, or on shelves that ran along the inside of the ship’s hulls.
SLAVERY IN THE NEW WORLD
A vast majority of the slaves brought across the Atlantic were imported into the Caribbean and South America. Only 6 percent of African captives were sent directly to North America. And yet by 1825, the U.S. had a quarter of the blacks in the New World. In the Caribbean, Dutch Guiana, and Brazil, the slave death rate was so high and the birth rate so low that they could not sustain their population without importations from Africa. Rates of natural decrease ran as high as 5 percent a year. While the death rate of U.S. slaves was about the same as that of Jamaican slaves, the fertility rate was more than 80 percent higher in the United States. The U.S, unlike other nations, had a self sustaining slave population for more than a century and a half. And the domestic slave trade in the U.S. continued even after the transatlantic slave trade was outlawed in 1808. (The Gilder Lehrman Institute of American History).
HALF OF ALL ENSLAVED INFANTS DIED IN THEIR FIRST YEAR OF LIFE
Children suffered very high mortality rates in slavery. Pregnant women were not given much of a break from their work in the fields. They still performed three-quarters or more the amount of work of non-pregnant women. Infant mortality was high, twice as high as southern white children. Half of all slaves died in their first year of life. A major contribution to this high mortality rate was chronic undernourishment. (The Gilder Lehrman Institute of American History).
DEATH AND DISEASE IN THE NEW WORLD
Death due to disease and malnourishment was also common, given that slaves were fed a low nutrition, purely starch based diet. Common symptoms of disease among enslaved populations included: blindness; abdominal swelling; bowed legs; skin lesions; and convulsions. Common conditions among enslaved populations included: beriberi (caused by a deficiency of thiamine); pellagra (caused by a niacin deficiency); tetany (caused by deficiencies of calcium, magnesium, and Vitamin D); rickets (also caused by a deficiency of Vitamin D); and kwashiorkor (caused by severe protein deficiency). Diarrhea, dysentery, whooping cough, and respiratory diseases as well as worms pushed the infant and early childhood death rate of slaves to twice that experienced by white infants and children. (The Gilder Lehrman Institute of American History)
No one knows the exact number of slaves who died in the New World. Yet in American Holocaust (1992), David Stannard estimates that some 30 to 60 million Africans died while being enslaved. He claims a 50% mortality rate among new slaves while being gathered and stored in Africa, a 10% mortality among the survivors while crossing the ocean, and another 50% mortality rate in the first “seasoning” phase of slave labor.