Everything has a risk in terms of bacterial/microbial transfer. That’s why you never go ass to mouth, or ass to vagina; the risk is high enough to be prohibitive.

Now, vaginas are just as microbially rich as any other part of the body, so the risk of something transferring and causing problems is definitely not zero.

Luckily, most vaginas are going to have similar species living in and around them. It’s all about balance. That’s how yeast infections and bacterial vaginosis happen. Something throws the balance off. It can be something as simple as a change in diet or as direct as a pathogen breaching the defenses.

But, it can be triggered by sex. Your dick doesn’t have the same proportions of microbes, and the exact strains of each are going to be at least a little different. It’s why a new partner can sometimes cause trouble even when nothing else is involved.

With sex, the microbes on the penetrating object are going much deeper than they’d ever get on their own. So the natural ability of the vagina to clean itself out is partially bypassed.

That’s where your question comes into play. If you go from one vagina to another, you will be carrying samples of their biomes between them. Assuming no STD involvement, the risk comes down to yeast, staph, strep, and trichomonas (though that is not necessarily going to be a standard part of the biome). Yeast in particular is on all of us, all the time. You can’t avoid it.

This means that if the strains of the three people involved are different enough, or their proportions are off enough, then it’s fairly likely that one or more of the group will end up with their personal biome off balance. When that happens, one of the species is going to outgrow the others, leading to symptoms.

That risk can’t be totally eliminated, even with bathing in between. That would make the risk absurdly low, but still non zero.

The good thing is that since vaginal secretions are going to be balanced for the person producing them, chances are also good that it won’t cause any problems. It’ll give a bit of cushion for the new body to adapt and rebalance.

Back in my days of doing group scenes and outright orgies, some women would just have yeast medications ready to go. It wasn’t like it would happen all the time, but it is part of the hassle of sharing fluids.

Barring a pathogen that’s more aggressive, or that doesn’t have good treatments, you’re looking at mild annoyance rather than danger. Assuming everyone gets tested for the dangerous stuff, it isn’t a major concern.

Also, pics or it didn’t happen (I’m 100% joking with that, unless that’s part of your fun)

I’m not a doctor so don’t go by me for a medical advice but I’m in a pretty poly community where we do such things too and just wanted to comment from my experience. But if you want to get a real medical answer go ask a professional of course.

Anyway that sounds really hot. And yes introducing someone else’s bacteria in an intimate area could cause problems. Everyone has their own little bacterial ecosystem and they are not always compatible. The first time I give head to a new girl I tend to get a cough too for a couple of days.

And there’s the STD thing too of course though you can easily test for that beforehand. In our community we test every 3 months (it’s free to do it that often)

But even without STD it could cause thrush etc. Annoying but usually easily solved with meds (one of those wax melting pills you can insert). It’s kinda up to them if they want to risk it I guess. Considering they are involved together too I do guess they already “mix bodily fluids” anyway though perhaps not quite as deeply.

Constantly changing the condom every time you switch is annoying too… Though as you’re planning to cum in her it sounds like you’re not planning to use one.

I’d go for the STD check in any case :)