Title: I Got Transported into Another World but Ended up Spreading Herpes to Everyone There
MC is a typical male isekai horn dog transported into a medieval fantasy world and receives an obscure blessing from a mysterious entity that he doesn't understand completely.
MC is part of the 50-80% of the world that has HSV (herpes simplex virus, HSV-1 are what a majority is infected with that is responsible for oral infection and ~40% of genital herpes), but is asymptomatic and doesn't know much about it.
MC opens his journey by visiting a town's brothel to quell his rampant sexual desires and ends up giving many of the female prostitutes there herpes.
The world's denizens have different immune systems compared to ours and do not produce the necessary antibodies to fight the foreign and new disease which shows much more virulent effects than what it would in normal cases, soon in a matter of weeks practically the entire town is infected with herpes as even sharing cups and towels spread the virus from the open sores of the townsfolk.
The MC figures out that something was wrong too late and knows its herpes from his world that he spread but could do nothing about it, he despairs in guilt and his isekai cheat awakens there: miasma coming from the townsfolk converge and congregate before him to form a beautiful naked girl, Herpes-chan, who our MC quickly falls for.
It turns out the blessing the MC received is from the Goddess of Pestilence, Agony, and Death; he is immune to all diseases of this world but has an ability to be a progenitor of every disease that his world has that can eventually manifest them into the physical world as beautiful girls. The more people he infects, the more the diseases have the potential to evolve making the disease/girls even stronger.
Everyone in the town now knows it was him that caused everything from the miasma trail and a witch hunt starts, MC and Herpes-chan runs to the refuge of another town only to find out that Herpes-chan uncontrollably infects the people around her and everything she touches with herpes.
The MC becomes infamously known as an apostle of the Goddess of Pestilence and Herpes-chan as her incarnation, existences that must be purged from the world.
MC decides to make the world his enemy for Herpes-chan's sake, there they will go on the journey to infect every living being with herpes to become the strongest.
A few harem members to be included:
Chicken Pox-tan: The cute loli daughter-like existence of the group, the MC already has an immunity to her disease.
Ebola-chan: The main attack force of the group, a bit yandere and loves MC but MC will likely die of Ebola if they have physical contact since she'll easily lose control close to him.
HIV-chan: The unlewdable white-haired loli STD.
Gonorrhea-chan: A seductress that plays with men but MC could be left infertile after being infected.
It would be a truly educational story about (loving?) STDs and other diseases.