HIV is transmitted by 1) having unprotected, penetrative anal or vaginal sex, or 2) sharing IV drug needles with other IV drug users.
Oral sex isn't considered a risk, with or without the presence of blood. It can be a risk for other STDs. If you have questions about those, post here: http://www.medhelp.org/forums/STDs/show/98
why cdc said the risk is low?
The CDC is very conservative.
Expert HIV physicians Drs. Hook and Handsfield state: “The risk of HIV, if your partner is HIV-positive, is around 1 in 10,000. That's equivalent to giving BJs to infected men once daily for 27 years before transmission might be likely.”
Neither recommend testing. I suggest that you give events like this the same concern as you might to being trampled in an elephant (which is probably none). If you have concerns about oral sex, then simply use a condom.
Thank you so much. Does bleeding gum increase the risk?