Thank you for the question and I feel sorry for your friend being infected with a sexually transmitted disease. First of all I would like you to know that a std is specifically transmitted through sexual activity therefore it is the prime reason for transmittance of the disease. Blood transfusion also transmits some of the std. the other thing is that the transmittance of a std depends on the type of std. there is a possibility that a std is transmitted when the blisters are present in the genitals and this may not be the case if the blisters are not present. Contrary to this, HIV can be transmitted at any time, even before it has evolved in the infecting person. Secondly, this may also be a result of her immune system. A person having a high immunity to a sexually transmitted disease has very less possibility of acquiring it. Further, the hygiene habits after the intercourse are another factor in this regard. similarly there are various other factors as well that would define her proneness to getting infected by this virus. Taking into consideration the second part of your question regarding the source of her std, I cannot let you know how she acquired it as this depends on the sexual activity of your partner. coming to the suggestion for your friend, I would like to say that it is high time that your friend and her boyfriend get treated by a doctor and avoid any sexual activity in the meanwhile. Hope this helps answering your question, good luck to your friend