Symbolism, Imagery, Allegory
Like it or not (and when you're dealing with syphilis, most people tick the "not" box), syphilis totally functions a symbol in Candide.
You might have noticed lots of references to original sin dotting the pages of Candide, right? Well, syphilis was doing a little European tour at the time Voltaire was writing Candide, and so, according to some scholars, Voltaire seems to be making a little religious analogy here: syphilis, like original sin, spreads through sex.
But not even an STD can make Pangloss shut up:
"Oh, Pangloss!" cried Candide, "what a strange genealogy! Is not the Devil the original stock of it?"
"Not at all," replied this great man, "it was a thing unavoidable, a necessary ingredient in the best of worlds; for if Columbus had not in an island of America caught this disease, which contaminates the source of life, frequently even hinders generation, and which is evidently opposed to the great end of nature, we should have neither chocolate nor cochineal. We are also to observe that upon our continent, this distemper is like religious controversy, confined to a particular spot. The Turks, the Indians, the Persians, the Chinese, the Siamese, the Japanese, know nothing of it; but there is a sufficient reason for believing that they will know it in their turn in a few centuries. In the meantime, it has made marvelous progress among us, especially in those great armies composed of honest well-disciplined hirelings, who decide the destiny of states; for we may safely affirm that when an army of thirty thousand men fights another of an equal number, there are about twenty thousand of them p-x-d on each side." (4.16)
Pangloss also alludes to the fact that syphilis is a disease that is a byproduct of colonialism—this is probably a sly comment by Voltaire on the sinful nature of empire-building.