Somewhere in a desolate forest lies a facility, hidden from the public. Although its funded and condoned by the government, its existence is to be kept secret from even most government officials. That is because this facility performs very unethical animal experimentation.
As a way to test recent advances in gene manipulation, this facility has taken the goal of trying to create a parasite of sorts that can make a female human grow male genitalia. The end-goal in this project is to make it possible for Lesbians to have children without adoption or insemination by a man, as well as making transition for female to male trans folk considerably more effective and satisfactory.
For these purposes, the scientists at said facility have taken to blood-sucking leeches due to their innate instinct to suck on lifeforms. Part of the method, is to modify the leeches DNA and add stem cell DNA to make the leech able to shed its old body, obtain DNA from the host and then turn itself into a fully functional penis. If course, the leech also has to somewhat automatically seek the right attachment spot for this purpose.
Although this would have sounded like nothing but pure science fiction out of a smut story, with how far gene manipulation and synthetic tissue production has come, the idea has at last become feasible.
The facility initially started out experimenting on rats. First, the leech attaches itself, before slowly turning into a skin sheath of sorts. The body then starts combing the given stemcell DNA with the hosts DNA and grows a penis out of this sheath, along with creating fully functional testicles below the penis. After many failed attempts, the scientists seemed to have finally figured out the exact adjustments they had to make to the DNA and started vastly expanding their experiments in scope and types of test subjects. They started growing penises on many animals, even managing to make unfitting penises grow on different animals.
After much trial and error, the facility managed to make human genitalia grow on many different animals, and the scientist felt the time had to come to start work on the truly risky part of it all - they needed to test their leeches on real, genuine human females. This was of course highly unethical, which is why the facility was an enigma to even the secret services, but it was a necessary step towards making the world a better place. In these experiments, the scientists would have to ensure that the grown penises would always be perfectly anatomically correct and even make it so that the modified DNA could be used to specify more about the penises characteristics itself.
However, that was when disaster struck. Despite the area of the facility having been know to not be seismically active, an unprecedented earthquake broke out and destroyed huge chunks of the facility. Most of the scientists were killed, along with their know-how and no agency could be contacted quickly enough - the damage had been done. Now the leeches, developed in the facilities with their still unrefined and unpredictable causes, had broken out and escaped in a nearby stream to be let loose on the unsuspecting society.