Somewhere between 500-800 million people rely on cassava root as their main source of carbohydrate. Incredibly it looks like many of them suffer from chronic cyanide poisoning as a result of improper preparation 
The quantity of cyanide depends on the cultivar, growing conditions and differences between the root and leaves of the tuber. The amount ranges from 15-1000mg cyanide per kilo of root.
Turning raw cassava root into a safe and edible food requires careful processing to reduce the cyanogenic glycosides. A combination of crushing/fermenting, plus drying seems best - some simple methods like boiling do very little to detoxify the root.
The levels quoted in the above table still seem very high, even after different forms of processing. The World Health Organisation sets the safe limit of cyanogenic compounds in cassava at 10mg/kg, which is much lower than those techniques listed.
Deaths from acute toxicity do occur, but more frequently the low level poisoning of people who eat poorly prepared cassava turns up all kinds of illnesses. These include neuropathies, cancer, congenital malformations, calcification of the pancreas and protein deficiency.
Konzo is the name of a paralytic syndrome caused by cassava poisoning combined with a low protein diet. It causes limb paralysis and is especially common in rural sub Saharan Africa. Improper preparation, over reliance on cassava and a lack of meat/protein.
Sub Saharan Africans diets in particular, with low iodine and protein, compounds with low but chronic levels of cyanide poisoning in areas where processing knowledge is not widespread, or new cultivars are being grown (or other reasons). The long term effects are very damaging.
A final example of this deadly combination is tropical ataxic neuropathy, a syndrome which seems especially common in cassava eating regions, esp where diets are vegetarian or low protein.
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