When the mouth and tongue are healthy and uncut, the chance of getting a sexually transmitted infection (STI) through oral sex is quite uncommon, though possible. Sores or scrapes in the mouth present a ready passage-way for virus or bacteria to enter the blood stream. Unless you're 100 percent sure of your partner's clean bill of sexual health, it's a good idea to use protection if any cuts, tears, sores or scrapes are present in the mouth (even tiny sores, like accidentally biting your tounge). Additionally, after you've had your wisdom teeth yanked, a root canal performed, or your dentures re-fitted, it's wise to stay away from unprotected oral sex (even if your partner is STI-free, your dentist would probably advocate treating your mouth gently after any of these procedures).
STIs like gonorrhea, syphilis, Chlamydia, HPV, and HIV can be transmitted through pre-cum or ejaculate. Herpes, especially when cold sores are present, can also be passed on through oral sex. Although these cases are very rare, transmission is possible.
Good providers do care a great deal, and are attentive to staying as healthy as possible as Naomi conveyed.