Leeches breathe through the skin. The digestive system contains a crop, or pouch, in which food can be stored for several months. One to four pairs of eyes are located at the anterior end. Individuals are hermaphroditic (hermaphroditism); that is, functional reproductive organs of both sexes occur in the same individual. Leeches are not self-fertilizing, however, for the sperm of one individual fertilizes only the eggs of other individuals. The eggs are laid in a cocoon, which may be deposited on land or in water. Development and growth are direct, without a free-living larval stage.

Land leeches await their victim in damp vegetation, poising one end in the air. The victim is often unaware that he has been bitten until blood is discovered running from the wound; blood flow may continue because of the anticoagulant still present.

Leeches that attack humans belong to the family Gnathobdellidae. Some species have been used medically for centuries; in Europe the use of leeches to drain off blood reached its height of popularity in the 19th century. Diseases commonly treated with leeches included mental illness, tumours, skin disease, gout, and whooping cough. A common treatment for headache was to apply several leeches to each temple and allow them to draw blood. Leeches have also been used to prevent excessive blood buildup following certain surgical procedures (e.g., reattachment of severed fingers).

In addition to H. medicinalis of Europe, the Algerian dragon (H. troctina) was used. Gnathobdella ferox was commonly used in Asia. After H. medicinalis was introduced into North America, it established itself there as a wild species. It grows to 10 cm (4 inches) in length and is green, with four to six brown stripes.

Other land leeches that attack humans are primarily of the genus Haemadipsa in Asia, the Philippines, the East Indies, and Madagascar. Leeches of the genus Philaemon are parasitic on humans in Australia.

Aquatic leeches, particularly Limnatis nilotica, may enter the body in drinking water. Some may enter the excretory openings of persons who bathe in infested waters. L. nilotica, which inhabits lakes and streams of southern Europe, North Africa, and the Middle East, attains lengths of up to 12 cm (4.75 inches), but younger, smaller specimens are most likely to enter the body. When ingested with drinking water they may first attach themselves to the linings of the nose or throat and then be inhaled into the lungs. A person infected with many such leeches may suffer from anemia resulting from blood loss. In external wounds secondary infection is more likely to occur than anemia. Leeches can cause suffocation and death of the host by blocking the breathing passages; in Asia, in particular, domestic animals commonly die in this way.