How can oral sex(a blow) cause hiv infection?

It cant.
The fact is although the semen contains HIV virus if infected, if it is swallowed it will be deaminated (destroyed) by the stomachs gastric juices which treat it like any other protein.
If your gums are bleeding they are bleeding outwards, so the virus still does not enter your bloodstream.

if any wound or bleeding happens in mouth it can infect to either or side.

whoever says you can't catch HIV from oral sex is WRONG. You abosolutely can!!! Look at my questions to see the answers I got for this same question. You need to use a condom if either of you haven't been tested. You can also catch other STDs from it. Granted, the change of transmitting HIV this way is significantly lower that though unprotected vaginal/anal intercourse, but it is still possible! Not every one knows this and the myth that oral sex is "safe" needs to be stopped.

there is no or anyother cases like this is ever received having hiv through a blow so be happy

Fully agree with Maggie. HIV can be transmitted by oral sex. Anyone who thinks contrary to this is absolutely WRONG.

Comparative to unprotected vaginal/anal sex the chances of transmission are less.

If a person infected with HIV is performing oral sex, blood from his/her mouth can enter the body of the person who is receiving oral sex through:
-urethral lining at tip of penis
-vaginal or cervix lining
-lining of anus
-open cuts or wounds

If an HIV infected person is receiving an oral sex, his/her blood, semen, pre-seminal fluid or vaginal fluid may contain the virus and the person who is performing the oral sex can get infected as the intact lining of mouth may allow the HIV to enter the body.

Risk of transmission increases if:
-one ejaculates in mouth of the partner
-cuts or wounds are present around mouth/throat
-person receiving oral sex has any other STD

PREVENTION:
=>Abstaining (not having) from sex is the most effective way. (but obviously not many ppl will like this idea !!)

=> Male who are receiving an oral sex should wear a condom

=>If you are performing an oral sex on a female partner:
- Use a latex barrier (dental-dam or simply cut open a condom to make it flat square)


SO PEOPLE WHO THINK THAT HIV CAN NOT BE TRANSMITTED BY ORAL SEX IS PUTTING THEIR AND THEIR PARTNER'S LIFE AT RISK!!

So transmit safe sex practice NOT AIDS !!!

cheers!