Syphilis
Syphilis, a disease transmitted both sexually and congenitally, most likely originated in the Americas before spreading to the rest of the world during the Columbian shuffle. The sexually-transmitted form of the disease, left untreated, progresses through three stages, each worse than the last.
#Potential Cultural Impacts
In small, nomadic hunter-gatherer groups, syphilis may break out, but cannot spread very far. Larger, more settled horticultural communities will have a harder time controlling the spread of this disease (as with other diseases). The spread of syphilis into a large community may even push their cultural mores toward monogamy and away from polyamory, as those with more sexual partners will have a higher likelihood of suffering from, and spreading, this infection.
#Fifth World Treatments
People of the Fifth World retain the memory of penicillin, though they now lack the ability to manufacture it from the penicillium mold. However, wizards and other healers may purposely grow penicillium mold on oranges, wait for blue-green mold to emerge, and transfer that to a clay container that they have made as sterile as they can manage (normally by boiling it or placing it in a hot fire for a good long time). Once they have grown more penicillin in the pot, they will add water and lemon juice (for acidity), then pour the liquid through a tightly-woven cloth into another container (which, again, they attempted to sterilize beforehand). The healer then gives this liquid to the patient to drink.
This technique bears a great deal more danger than modern-day penicillin does, as a healer can never entirely know what other molds hen may have fed the patient. Diarrhea, vomiting, and even death may ensue. Responsible healers will reserve such a risky treatment only for late stages of terrible diseases such as syphilis, and only after making absolutely sure the patient understands the risks.
Another risky treatment consists of purposely infecting the patient with malaria (usually by encouraging hen to spend time in malarial swamps) in the hope that the prolonged high fever will cure the patient's syphilis. (Fourth Worlders found this treatment shortly before discovering the safer penicillin.) People of the Fifth World tend to simply avoid swampy areas with high risk of malaria. However, the cinchona tree -- the source of quinine, an effective treatment for malaria -- grows in the tropics. As the world warmed after the collapse, and people spread the seeds of tropical plants to former temperate and even frigid zones, they made sure to sow the seeds of the cinchona tree so that people everywhere could treat malaria. Thus, Fifth World healers can give malaria patients cinchona bark tea. However, this treatment still comes with quite a bit of risk. Responsible healers will only resort to this in cases of neurosyphilis.