system
Given the following post, respond as if you're commenting on Reddit:
human
How does STD Transmission work?

In recent months, I've become curious about how STD's such as HIV, gonorrhea, herpes, etc are transmitted between partners. Withholding transmission mediums such as IV needles and the like, how does STD transmission occur during sex? More specifically, why are some STD's (such as HIV) more likely to be transmitted via (unprotected) penetrative sex than oral sex? My original intuition is that the infected bacteria only need to find an entry point into the body and at that point the person will become infected. Seemingly, this would make oral sex just as dangerous as penetrative sex. Obviously I am here to learn from individuals much smarter than myself so I look forward to your input. Thanks in advance.
gpt
Infectious bacteria and viruses don't just infect "the body", but usually have specific body parts or cell types that they can survive and grow inside of. You can think of it similar to how animals can be the top predator in certain ecosystems but would be unable to survive in others. A polar bear from Canada could probably do pretty  well in Greenland, but if you put it in Egypt it wouldn't survive.

Similarly, some organisms can infect the cells of the reproductive tract, but can't survive in other areas. We call these "sexually transmitted infections" because this is the main way that they get spread, but it isn't the only.

HIV can survive in reproductive cells and the rectum, but is quickly killed within the mouth. Its ultimate target is immune cells, so you can also get infected if it is directly put into the blood stream. So oral sex is not a risk factor for HIV, but vaginal sex, anal sex, and needle-sharing can lead to infection.

Gonorrhea infects the reproductive tract, but it is also pretty good at surviving inside the throat. So oral, anal, and vaginal sex are all possible of transmitting it. Unlike HIV, it usually doesn't survive anywhere else in the body such as the blood, so you are unlikely to catch it via sharing needles. 

Conversely, viruses that cause the common cold can easily survive in the throat, but they would not be able to survive in the reproductive tract. So kissing can transmit a cold virus, but oral sex is not going to.

Hope this answers your question!