But flesh with the life thereof, which is the blood thereof, shall ye not eat. -- Genesis 9:4
And whatsoever man there be of the house of Israel, or of the strangers that sojourn among you, that eateth any manner of blood; I will even set my face against that soul that eateth blood, and will cut him off from among his people. For the life of the flesh is in the blood. ... Therefore I said unto the children of Israel, No soul of you shall eat blood. ... And whatsoever man there be of the children of Israel, or of the strangers that sojourn among you, which hunteth and catcheth any beast or fowl that may be eaten; he shall even pour out the blood thereof, and cover it with dust. For it is the life of all flesh; the blood of it is for the life thereof: therefore I said unto the children of Israel, Ye shall eat the blood of no manner of flesh: for the life of all flesh is the blood thereof: whosoever eateth it shall be cut off. -- Leviticus 17:10-14
Many Jehovah's Witnesses have died because the governing body considers blood transfusions to be "eating blood." And although their view on this has recently changed to allow some use of blood, the GB is careful not to explain its new, complicated rules on blood to its followers.