On Oral Sex and STD

This task was given to me because I was curious if STDs can be acquired through oral sex. Asking from a forum in Fetlife, they pointed out that I dont need to swallow semen to have it, because pre-cum can already do it.

How are STDs acquired? STDs are acquired through transfer of body fluids either sexually or non-sexually as in open sores. This I've learned and many other things in http://www.emedexpert.com/tips/std-facts.shtml

Since my primary concern (then) was ora sex, what are the benefits and diseases you can have from oral sex? a wiki page gives a very good picture

Reading on what tests can determine if one has STD, there are a lot of tests done on a lot of STDs but most of them are through urine and blood tests