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Leeches were used in medicine from ancient times until the 19th century to draw blood from patients. In modern times, leeches find medical use in microsurgery, while hirudin is a valuable drug for some blood-clotting disorders.
Leeches normally carry parasites in their digestive tracts, which cannot survive in humans and do not pose a threat; however, bacteria, viruses, and parasites from previous blood sources can survive within a leech for months. Nevertheless, only a few cases of leeches transmitting pathogens to humans have been reported.
Some 680 species of leech have been described, of which around 100 are marine, 480 freshwater and the remainder terrestrial. Among Euhirudinea, the true leeches, the smallest is about 1 cm ( 1⁄2 in) long, and the largest is the giant Amazonian leech, Haementeria ghilianii, which can reach 30 cm (12 in).