They are gross, they attach themselves to your body if you happen to be walking through a rainforest and, worst of all, they suck your blood. But while leeches are in the same line of business as vampires, they can actually work medicinal wonders.
The use of leeches goes back more than 1,800 years, when the Greek physician Galen decided they were a good way to suck away "bad" blood. Surgeons in the 1800s were also quite fond of using leeches in bloodletting, which was thought to cure anything from headaches to gout.
Medical practitioners have returned to leeches in recent years, following the discovery that they can help to restore blood circulation to grafted tissue and re-attached fingers and toes.
The leech removes congested blood and allows normal circulation to return to the tissue, so preventing gangrene. They produce a number of substances, including an anticoagulant that allows bleeding to continue up to 10 hours after the leech has fallen off. They also produce their own local anaesthetic, which makes their application painless.
Biopharm has been supplying medicinal leeches since 1812. Its farm near Swansea in Wales, home to more than 50,000 leeches, supplies hospitals and laboratories around the world.
Scientists have identified several compounds that can be developed from leech saliva. Their anticoagulant and clot-digesting properties make them potentially useful in the treatment of cardiovascular diseases.
Leeches can be "milked" for their secretions without being harmed, but a synthetic version of one of these compounds - called hirudin - went on sale last year. Without it, 10,000 leeches would be needed to treat just one patient.
So, in the unlikely event that you slice a finger off and have to get it sewn back on, don't be alarmed if the doctors start talking about leeches - not everything that sucks blood is bad.