The Resident Evil series of movies and video games regularly features grotesque monsters created from the exposure of humans and animals to the T-virus/G-virus/C-virus.
Obviously the way this is portrayed is unrealistic, with a mad scientist injecting themselves with a vial and immediately turning into a monster, but I'm curious about how far back this kind of thing goes in fiction. I know of examples in superhero comics.
I know real-life experiments with inserting DNA into cells via viruses were happening as far back as '72, and viral gene therapy is an ongoing area of research, but I'm curious as to whether or not fictional instances of viruses mutating people precede these instances of real-life research.
The prototype for these kinds of stories is probably Dr. Jekyll and Mr. Hyde, with a 'serum' that transforms a man, but I'm more interested in explicitly monstrous transformations brought on by a virus.
EDIT: There have been some really interesting answers, but nothing that was quite exactly what I had in mind, so I'll try to narrow my question.
I know that at least in Japanese superhero media, there is at least one instance of what I'm thinking of: In the Super Sentai series Liveman, one of the villains injects himself with a virus of his own creation to become more powerful.
I'm not sure how much of an influence this would have had on the developers of Resident Evil, but I suspect that if there was one instance of this, there were more that I'm unaware of.
So to narrow the question down some: were there any instances of viruses mutating people in monstrous ways that would have been accessible to, and may have influenced, the developers of Resident Evil?