How infectious is syphilis?

In a study of the sexual contacts of patients with primary and secondary syphilis, 65 of 127 (51%) contacts at risk developed syphilis. There was no significant difference between figures for homosexuals (48/98, 49%) and for heterosexuals (17/29, 58%). Our findings are similar to those of the prepenicillin era, but the question, Why are so few contacts infected? remains unanswered.