Leeches feed on blood. Having attached itself to the host animal, it pierces the skin and injects an anaesthetic to hide the pain of its bite so that the host does not find the leech and remove it, and an anticoagulant chemical, which prevents the host's blood from clotting whilst the leech feeds. The length of time a leech may feed seems to vary. One surveyor, as an experiment, allowed a leech to feed on him and it fed for 83 minutes. Another, presumably less-hungry individual, fed for just 25 minutes. During a meal, it may extract 15 millilitres of blood, which can increase the size of the leech by anything up to 11 times its normal body dimensions. A leech's meal can sustain it for over six months, but it may also have to wait many months between feeds. During this time, they can digest their own body tissue to avoid starvation. Leeches find their host animals by detecting disturbance in the water, and they can prey on small creatures as well as large. A frog or a newt, for example, can die from excessive blood-loss following an attack by a leech. Leeches may also behave as predators on some species of fish such as sticklebacks, as well as on great-crested newts and marsh frogs.
Leeches are often found in the nests of birds such as moorhens, and seem to use them as shelter as well as finding a food source. Dismantling old nests can be a method of surveying for leeches. The eggs are laid in a spongy cocoon on damp ground and, at Dungeness in Kent, they are associated with the roots of willowherb. On nearby Walland Marsh, the eggs have been found in damp turf on close-grazed sheep pasture. Humans are susceptible to parasitism by leeches but, apart from feelings of disgust, most suffer no ill effects. However, medicinal leeches have been used for removing 'bad blood' from human patients for hundreds of years. Although this practice fell into abeyance by the beginning of the 20th century, leeches are once again being used to restore blood circulation following tissue grafts. Leech saliva may also prove helpful in the future, too, as it apparently contains antibiotics as well as anticoagulants that may prove useful in surgery.