Eli5: if there are a ton of harmful bacteria in every persons mouth, why is it relatively safe to give/receive oral when genitalia is so sensitive to foreign bacteria?

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Eli5: if there are a ton of harmful bacteria in every persons mouth, why is it relatively safe to give/receive oral when genitalia is so sensitive to foreign bacteria?

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It really depends on what is there. The human vagina itself is acidic and has its own bacteria flora, so it’s not like adding oral bacteria to a clean petri dish. Some bacteria can be pretty environment specific and there’s bound to be competition in the human vagina. It’s not going to be a 1:1 transfer. A bacteria used to a certain pH, moisture, and substrate might not do so hot in certain areas. Also I am not too sure the human mouth is filled with harmful bacteria. What do you mean by this?

But for STIs like chlamydia that hide inside our mucous membranes and get inside our cells, that can transfer from oral to genital because both have mucous membranes. But this is not a native bacteria for humans.

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u/mikeroscopic_ answered wonderfully. To expand on their point, a human bite is very dangerous if it penetrates what’s called our “1st line of defense”. This constitutes our skin, hair, nails, and our outer microbiome (maybe). (https://pubmed.ncbi.nlm.nih.gov/35105981/#:~:text=The%20innate%20immune%20system%20acts,recognition%20and%20the%20resultant%20inflammation)[kinda related]

If it gets past that, it’s basically one of us, and up to our second line of defense, our immune system.

For reference, imagine a bad oral germ is Bob. Bob tries to get into our house through the front door. But we locked our front door, so he tries to break it down. Unfortunately for Bob, he’s a tini boi, so he crumples against our door. Defeated, he leaves to think of another way to get in.

Bob returns, and tries to enter through our basement! For whatever reason we open the cellar doors and let him in, but Bob faces a decision: break through the walls, or sneak up the stairs? Bob decided to sneak up the stairs. Unfortunately for Bob, we thought he might try this, so we put anti Bob poison on the stairs. Bob fucking dies.

Unfortunately for us, Bob has cousins. Bob’s cousins are just like Bob, but they’re hillbillies connected to a crazy person, and they somehow get a first class ticket straight into our upstairs hallway. Bob is now in the house, so it’s time to settle this once and for all.

For reference:
Bob = Bacteria

Front door is the mouth / skin

Basement = anus / vagina
+ colon / vaginal canal

Walls / stairs = ruptured colon / vaginal canal
Through GI / GU Tract

Upstairs = Vagina (internal). Relatively good at being inhospitable to intruders

Cousins = the same oral germ, via a bite that penetrates skin

Edit: misspelled name

Another factor is that you’re not actually breaking the skin. Any bacteria left by the mouth will get washed away by the shower that will hopefully be coming relatively soon.

1. The areas that are most likely to have foreign bacteria have the best protections against them. Basically any area where foreign objects can easily enter your body. This includes your mouth, nose, eyes, ears, genitals, anus.

These areas will be *Much* better protected than a random cut you get, your body is absolutely not prepared to have a foreign object enter your kidneys. Now that doesn’t mean it’s safe to stick foreign things in these areas, but it’s saf-*er*. Also some of these areas – while better protected – are still more delicate, and thus more susceptible to infection, eg. your eyes.

2. It’s not safe. “*Relatively* safe” means you’re less likely to get an infection in your mouth or your genitals than if you somehow got it in your lungs, but it’s absolutely not safe.

If you’re not in a commited relationship you should always use condoms. In all-girls schools they teach the girls to use rubber gloves when engaging in same-sex activities to avoid spreading infections – I don’t know if any of them listen but this is the best medical advice. For oral sex on girls I don’t know if there is a good answer (*if anyone knows feel free to chime in*).

3. This will somewhat contradict my previous 2 points, but please don’t take away that there is no danger. The third point is that Human Beings have developed immune systems to deal with infections. Chances are you have some kind of infection or antigen causing you problems right now, but as long as your immune response can handle it you won’t even notice. You only start to feel sick/sore/feverish when the infection/virus/whatever starts to overwhelm your immune system. Once again though don’t take from this that you’re invincible. Mostly people are fine, but sometimes they end up in hospital with a story too embarrassing to tell their boss on Monday.