A big difference about outing someone with syphillis vs with AIDS is back then it was thought by a lot of folks that AIDS was only transferrable by anal sex, i.e. you would also be outing them as probably gay/bi.
Which was also much more dangerous to be outed as back then.
A post from Gregory Cochran's and Henry Harpending's excellent blog West Hunter.
The commenter Ron Pavellas adds:
The Wasserman Test.