But to really understand the risks of such transmission, you need to know more about the basic transmission of STDs . We now think of them in two categories, those that are predominately transmitted through sexual activity, and those that are occasionally but not predominately transmitted through sex. In the former group would be the old bacterial STDs, gonorrhea , syphilis , chlamydia and others, and the more recently-important viral STDs, HIV , hepatitis B , genital warts , herpes , and others. You must also consider the efficiency of the transmission in each bodily fluid.
Male semen is the most efficient transmission fluid.
The viruses and several of the bacteria are present in large numbers, and ejaculation, whether into a vagina, a rectum, or the mouth, carries a high risk of infection. It is for this reason that condoms are such an important method of preventing infection. Vaginal secretions are a less efficient transmission fluid, so male to female, or male to male, infection is easier than female to male or female to female, whether through intercourse or oral sex. But even if less efficient, there obviously is a lot of female to male transmission. Saliva is the least efficient transmission fluid, but that doesn't mean it can't happen.