usually the transfer comes from having contact or exposure with infected person through there mucus membranes, genitals, blood..
and other body secretions but except the tears, sweat
and saliva..
one thing that can lessen having STD is using condoms,
though it only lessen the possibility but they can acquire it also if they tried to have oral sex.
the only thing that can prevent having STD is having a monogamous relationship. not trying to have sex with other person you didn't know.