Rogues have the Disease Use talent, which allows them to inflict the Filth Fever disease on targets. He simply has to collect some filth somewhere and apply to his weapons.
A 3rd level antipaladin is immune to the effects of diseases, but not immune to being afflicted by disease, which means he is a carrier who will never show signs of being disease (not without spells, at least). So, he could obtain this disease from normal means, and spread it around to others, while charging to cast Remove Disease obtained from another class (inquisitor, cleric, witch, etc).
Any class with access to the Contagion spell can inflict one of the following diseases to touched creatures: blinding sickness, bubonic plague, cackle fever, filth fever, leprosy, mindfire, red ache, shakes, or slimy doom. Of course, there are stronger alternatives aswell: Epidemic and Plague Storm.
As an alternative, you have the system of crafting Drugs from the Alchemy Manual Player Companion, which can be made using Craft(Alchemy). Those are canon in Golarion, the book even introduces a few drugs from Katapesh.
As for mundane ways of producing a disease, you would require something like Profession (Pharmaceutical Scientist), which doesn't seem to fit most medieval settings. But i could see it working with biologist aswell.