Why is the manchineel tree dangerous?

Manchineel - is dangerous because its sap and fruit contain poison. Its other name is Hippomane, which comes from the Greek and means "causing horses to run mad". The tree is a member of the Euphorbiaceae family and grows in tropical America, producing a crop of acrid, bitter apple like fruits which drop spontaneously and carpet the ground beneath it. The sap is white and highly caustic, so that a drop on the skin produces a burning sensation and raises a blister. It used to be believed by many that to sleep beneath the tree meant certain death. But the great 18th Century naturalist Nicolas von Jacquin "reposed under it for hours at a time without inconvenience. The wood has often been used for furniture as it is beautifully patterned in brown and white. Before felling the tree by hand, workmen light a fire round the trunk, so that the sap thickens and does not run down the handles of their axes.