What was the first story wherein an artificial plague is created for therapeutic reasons?

Question

In Miri from STOS, I do not think specifically an artificial virus or artificially-modified virus is mentioned, but Wikipedia says, "The party find a medical research laboratory and look through documents for clues to the disease, discovering that it is a side effect of a life-extension experiment."

Now, this is not quite explicitly saying that a virus or I guess bacteria as the other possibility was deliberately created to spread the therapy -- perhaps even the experiment just ended up modifying a microorganism which does not have any value but is responsible for illness which the life-extended children are protected from. I believe this story was novelized or a novel about the same planet was written and perhaps the novel goes into this.

I only mention Miri because it is a natural guess as the first story to feature a therapy spread perhaps via virus but not necessarily.

Of course, I am Legend with Will Smith is explicitly the chicken pox virus changed to fight cancer. But I suspect this is not the first story in which this therapeutic idea is presented. Note that the book has no concept whatsoever of a virus or bacteria (it is a bacteria that causes the disease) being modified for therapeutic reasons -- I think Matheson has it as naturally occurring. In The Omega Man, it is germ warfare and maybe the virus/bacteria has been modified; but again, not to spread anything therapeutic.

My guess is, the first story in which a contagious organism/virus is created to spread a beneficial therapy was written after the discovery of DNA's structure. I think prior to that, viruses were considered too mysterious for any writer to envision changing them. I would of course welcome being wrong.

Summary of question: First story (or movie, etc.) in which a disease is modified to be beneficial?

Answer 1

Probably not the first case, but the 1970 novel Son of Kronk (later just Kronk) involves a sexually transmitted disease that causes pacifism.

The protagonist, Gabriel Crome, book artist, is sharing a bottle of vodka with a raven and contemplating suicide when he has the opportunity to save the life of Camilla Greylaw who has similarly arrived to commit suicide. After convincing her to give life a chance, he learns that her scientist husband has infected her with a virus named P939 which is a sexually-transmitted disease that removes all aggression from those it infects. They eventually decide that it is their destiny to spread this disease among the world and thereby bring about world peace.

Admittedly, they didn't know about the side effect,

that the next generation born from those infected are extremely violent. They only learn this after thoroughly spreading the infection, and realizing that the utterly pacifist humans will be easy prey for the second generation of animals already infected... let only their eventual progeny.