Red leeches were small creatures not dissimilar to lobsters with large black eyes, sucker-like mouths and two tentacles covered in barbs. They lived attached to other creatures, though they required excess salt to survive. If the host was dying, the red leech would leave and find a new one.
The red leeches could produce a deadly poison. Depending on the dosage, the poison could cause blindness, paralysis or death. In some cases, it would cause the person's skin to become red, waxy and hard. (TV: The Crimson Horror)
The red leeches were an ancient species; the Eleventh Doctor knew them to live in the Jurassic period, while Madame Vastra believed they died out 65 million years BC. They were known to the Silurians, who considered them their most virulent enemy.
At least one of the red leeches survived into the Victorian era and was discovered by Winifred Gillyflower who named it Mister Sweet. She created Sweetville, enticing people to move there and then paralysing (or "preserving") them by dipping them into vats of diluted red leech venom. She then planned to send up a rocket containing pure red leech venom, which would explode and shower the population with poison. The people of Sweetville could then be revived to repopulate the world. The preservation process did not work on all of those upon whom it was attempted, resulting in "rejects" (including the Eleventh Doctor), who displayed the symptoms of those exposed to pure red leech venom. The bodies of these "rejects" were then disposed of in nearby rivers. After Gillyflower was killed by Strax, the red Mister Sweet was killed by her daughter, Ada before the Doctor could return him to the Jurassic era. (TV: The Crimson Horror)
Behind the scenes
The red leech is a reference to "the repulsive story of the red leech", a Sherlock Holmes adventure mentioned by Dr Watson in "The Adventure of the Golden Pince-Nez".