Data- It's Possible

"We find no study that addresses sexual transmission of Bb among humans; conversely, we find no study supporting that it does not occur. Inferential data, however, suggest the possibility of human sexual transfer. The data come from sound veterinary studies (96,98,115), the finding of Bb in human semen and breast milk (128,129), and by similarity to Treponema pallidum where sexual transfer is abundantly documented (117,130,131)."

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http://www.ilads.org/files/harvey.pdf