A Leech is a predatory worm which can be found in freshwater and on land, depending on species. They have a relationship with humans and other animals in which they bite and clamp onto the skin to draw blood - they also have the potential to carry and transmit bacteria and viruses.
Dr
James Marcus of Umbrella used regular leeches in his research to create the
t-Virus by injecting them with a sample of the Progenitor virus which caused them to mutate, growing larger, more intelligent and more aggressive. The transformation also altered the Progenitor virus within them, becoming what's known as the
t-Virus. These Progenitor Leeches became the main component of Dr Marcus' work at Umbrella.
Eventually, thousands Progenitor leeches were bred by Dr Marcus and they fostered a unique relationship with him - they had the ability to learn and work together. Hundreds of human test subjects were used and later destroyed in all of his experiments with the leeches, including children - this caught the attention of other Umbrella employees which caused a lot of concern, leading to the company constantly attempting to stifle his work.
After a while, the Progenitor leeches were able to join together to form a single larger body - and eventually they were able to become a facsimile of Dr Marcus himself due to their fascination with him - and this entity was known as a
Mimicry Marcus. The leeches secreted a kind of fluid in order to both hold the shape and also give the appearance of a more contiguous object. In close contact, the creature appeared slimy and rubbery, owing to the fact that there was no skeleton like a real human body would. Thus, the arms and legs appeared to stretch at times, and if enough damage was caused to the leeches, the structure would fall apart and the remaining leeches would attempt to escape.
Dr Marcus was eventually killed by Umbrella Security after his research became too bothersome to the rest of the company. His body was disposed of in the same area many his leech specimens were also disposed of, and subsequently one of the leeches identified his body as a food source and consumed it. This gave rise to a unique mutant named
Queen Leech.
Queen Leech had consumed Dr Marcus' brain and body to attempt to restore itself, however in the process it gained his memories and ideas. After a decade of evolution and contemplation of its new found identity, it learned how to take the shape of Dr Marcus, believing themselves to be the man himself - and due to Dr Marcus' strong Christian beliefs, also believed itself to be an incarnation of Jesus Christ. In actuality, it was a large, twisted monster with tendrils and a leech-like mouth, its human form was something which had to be maintained. It was eventually destroyed by
Billy Coen by gunfire, and indirectly by Dr
William Birkin who destroyed the facility it was kept via a self-destruct mechanism.
After this incident Leeches were no longer used by Umbrella in any research other than utilising them as a vessel to create and hold the
t-Virus.
