I did a book of diseases for the DMsGuild:
http://www.dmsguild.com/product/173091/5MWD-Presents-Diseases--Poisons
One of them was this bad boy:
Great Pox
Also known as "reveler's bane", this disease is common
among prostitutes, sailors, and amorous adventures.
Despite rumours to the contrary, elves are not immune
to this disease. Elves and half-elves do not display
symptoms of great pox or suffer negative effects, but can
readily spread the disease to other races.
When a creature has sexual contact with an infected
creature they must succeed on a DC 15 Constitution
saving throw or become infected. If the infected creature
is symptomatic then this saving throw has disadvantage.
While there are many proposed methods of preventing
infection, none of these have any actual effect.
The disease lies dormant for a variable length of time,
and creatures can be asymptomatic for many weeks.
Symptoms of the reveler's bane appear after 2d6 weeks
but the infected creature is contagious after 2d4 days.
Symptoms include a rash across the hands and groin that
eventually becomes painless ulcers.
After each long rest, the infected creature takes 5
(2d4) poison damage and must make a DC 13
Constitution saving throw. On a failure the poison
damage increases by 2 (1d4). On a successful saving
throw the damage decreases by 2 (1d4). When a
successful saving throw reduces the damage below 1 the
creature symptoms go away, but the creature remain
infected and must continue to make saving throws at the
end of each long rest. After three consecutive failures,
symptoms return, and after three consecutive successes
the creature is cured.