Reading the Wikipedia entry on pastoral, I wondered whether it had been targeted for an outré prank. But no. Syphilis, the disease, really does take its name from Syphilus, a character in a pastoral poem:
A harsher note was struck in Girolamo Fracastoro's 1530 poem Syphilis, sive Morbus Gallicus ("Syphilis, or the French Disease"), in which Syphilus ("pig-lover"), a typical pastoral name for a shepherd, is stricken by the disease syphilis that takes its name from Fracastoro's poem. Fracastoro's poem contains the first recognisable description of the symptoms of syphilis; today, far too few contemporary physicians announce their discoveries in verse, pastoral or otherwise. Fracastoro has Syphilus the shepherd catch it for having offended Apollo, a somewhat unusual method of infection. Fracastoro's Latin poem was much admired in its day; it was translated into English heroic couplets by Nahum Tate:» Pastoral (from Wikipedia)
A shepherd once (distrust not ancient fame)
Possest these Downs, and Syphilus his Name;
Some destin'd Head t'attone the Crimes of all,
On Syphilus the dreadful Lot did fall.
Through what adventures this unknown Disease
So lately did astonisht Europe seize,
Through Asian coasts and Libyan Cities ran,
And from what Seeds the Malady began,
Our Song shall tell: to Naples first it came
From France, and justly took from France his Name