Hi @iabhi1227! Great questions 🙂
To answer your first question, I’ll explain in a little more detail how HIV is transmitted.
1. The person you have sex with has to be HIV+ and have a detectable viral load (undetectable means almost no risk of transmission) AND
2. You have to be in contact with certain body fluids: semen, vaginal fluid, anal fluid, blood, or breast milk (saliva, sweat, & tears don’t transmit HIV) AND
3. There has to be enough fluid & it has to be fresh (HIV doesn’t survive outside the body for long). AND
4. There has to be a way for that fluid to get into your blood stream.
So… if a woman – whether she’s a sex worker or not – performs oral sex on you, you’re only exposed to her saliva which doesn’t transmit HIV. That means there’s no way of catching HIV (though you can get other STIs!).
If you perform oral sex on her, you’re exposed to her vaginal fluid in your mouth, but your mouth doesn’t provide easy access to your blood stream. If you have a significant open wound in your mouth, that can become a pathway for HIV. That means there almost no risk of HIV, unless you have this type of wound.
To answer your second question, condoms are a great way to decrease your risk of all STIs. But how well they work depends on how the STI itself is transmitted. Condoms are super effective against STIs that are transmitted though body fluids (eg HIV, chlamydia, & gonorrhoea). Condoms significantly decrease the risk of STIs that are transmitted on the skin (eg HPV or genital warts & Herpes).
Does that clear it up for you? 🙂