It is a common question. Our straight forward answer is: NO.
None of the common sexually transmitted and blood borne infections (STBBIs) like syphilis, chlamydia, gonorrhea or herpes can spread this way. Bacteria or viruses that cause many STBBIs cannot survive outside the body, they start to die with the slightest change in their environment. Herpes is known to transmit skin-to-skin but not from surface-to-skin. Similarly, Hepatitis and HIV cannot be transmitted from surface-to-skin as well. Evidence indicates that toilet seat poses no risk for transmission of STBBIs.
We are glad that you asked this question and hope our answer was helpful.