One of the most common STDS that are both given and received in a blow job is Herpes. A cold sore is a form of Herpes. It is not the same type of Herpes usually considered as an STD but if the person giving the BJ has a BJ or oral sex the virus can be transmitted to the person receiving oral sex.
Best advice I can give concerning cold sores. If you or your partner have a cold sore; refrain from kissing or oral sex until it is healed and gone. Even though it is a different type of the Herpes virus you do not want it on your private parts. [ adviceman49's advice column | Ask adviceman49 A Question ]
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