I guess the song was written long before google but that's what the song was about..
'If'n it was'n fo' Cotton eyed Joe, I'da bin married a long time ago..Where do you come from, where do you go? Where did you come from Cotton-Eyed Joe'..
Not hard to understand why they had no idea about venereal disease or it's cause even though it was well known and 'treated' in the cities...