HIV/Aids
According to the San Francisco Aids Foundation, oral sex is "low risk" when it comes to transmitting HIV. Experts suggest there is no risk of contracting the disease when receiving oral but if you are performing fellatio, although unlikely, there is an increased risk if you have cuts or sores in your mouth, have an STI or have had recent dental work. Use a condom or pre-exposure prophylaxis to further reduce risk. There is no known cure but treatment is available to alleviate symptoms.
HPV and HSV (Type 1 and Type 2)
You can develop HPV and HSV from both giving and receiving oral sex. If your genitals come in contact with the mouth of an infected sexual partner you are at risk of catching genital herpes or genital warts. Similarly, if you perform oral sex on a sexual partner who has genital herpes or warts, they may transmit the infection your way. Active sores will increase your risk. There is no known cure for HPV or HSV.
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Gonorrhoea
Gonorrhoea is one of the more common STIs that can be passed by oral sex. Formerly known as 'the clap', the infection causes discharge from the genitals and pain while passing urine. As a result, the bacteria is found in bodily fluids and therefore can be transmitted during unprotected sex. - PIV, Anal and Oral. Gonorrhoea is treatable.
Syphilis
Similar to HPV, syphilis can be transmitted by coming into direct contact with an infected area, known as a chancre. The infected person usually has painless sores that heal naturally, but if left untreated, it can causes rashes or possibly even serious damage to the brain, nerves and eyes. Syphilis is treatable.
Chlamydia
Similar to other STIs, Chlamydia is a bacteria infection that can be spread through genital fluids and sexual contact. The risk of transmitting through oral sex is extremely small. Like gonorrhoea and syphilis, symptoms include pain while peeing and abnormal discharge.