[6] Medicinal leeches are hermaphrodites that reproduce by sexual mating, laying eggs in clutches of up to 50 near (but not under) water, and in shaded, humid places.

The preferred habitat for this species is muddy freshwater pools and ditches with plentiful weed growth in temperate climates.

Over-exploitation by leech collectors in the 19th century has left only scattered populations,[3] and reduction in natural habitat through drainage has also contributed to their decline.

Another factor includes the replacement of horses - medicinal leeches' preferred host species - by motor vehicles and mechanical farming equipment, and the provision of artificial water supplies for cattle.

The largest, located at Lydd, England, is estimated to contain several thousand individuals; 12 of these areas have been designated Sites of Special Scientific Interest.

[11] The next recorded uses of leeches in medicine come in the last few centuries BCE, by the Greek physician Nicander in Colophon[5] and in the ancient Sanskrit text Sushruta Samhita.

[12] Leech therapy is mentioned a few hundred years later in Shennong Bencaojing, a 3rd-century CE book of traditional Chinese medicine.

[citation needed] These sources indicated leech therapy for a wide variety of ailments, including edema,[12] "blood stasis",[13] and skin diseases.

Leeches, by removing blood, were thought to help with these kinds of conditions — a wide range which included illnesses like polio and laryngitis.

[17][20] Other clinical applications of medicinal leech therapy include varicose veins, muscle cramps, thrombophlebitis, and osteoarthritis, among many varied conditions.

[5] The most common complication from leech treatment is prolonged bleeding, which can easily be treated, but more serious allergic reactions and bacterial infections may also occur.