Mistletoe is called the "Vampire Plant" because it can probe beneath tree bark to drain water and minerals, enabling it to survive during drought (see vampirism). It grows on deciduous trees of nearly any sort and can prove fatal to them.
The leaves and young twigs are the parts used by herbalists, and it is very popular in Europe, especially in Germany, for treating circulatory and respiratory system problems as well as for tumors, even malignant ones.
Mistletoe is spread by birds (especially the Mistle thrush) who eat the berries; the seeds are excreted in their droppings and stick to twigs. The word may be related to German Mist, another word for dung; but Old English mistel was also used for basil.
Mistletoe figured prominently in Norse mythology - the god Baldur was killed with a weapon made of mistletoe - and Celtic mythology (whence the modern Western custom of kissing under bunches of it hung as holiday decorations) and in Druid rituals. It was considered an antidote to poison, but contact with its berries produces a rash like poison ivy rash in people who are sensitive to it (as many are), so the whole plant came to be thought of as poisonous.