What happens if you use a vibrator anally but you're not clean in there?

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Would this be dangerous somehow?
I mean if there should be fecal matter in there and then you insert a vibrator then could
this cause problems? Could the vibration somehow cause the fecal matter to get into the
mucous membrane and then cause an infection?

There's always some stuff up there. Faecal matter doesn't usually cause infections, despite what you would think--people with haemorrhoids get bleeding when wiping but don't usually get infections.

A vibrator should not cause damage to the membranes. That is why they have ends that are smooth. I think the greatest danger comes from people using non-commercial objects that might have rough edges or splinters.

But lets say a person has rough anal sex and she didn't use a douche before and there is fecal matter
inside then what if the fecal matter gets totally rubbed into the membranes during the sex? Would this not be unhealthy or dangerous somehow? I mean when you have a bowel movement then the stuff also moves around in there but it's not comparable to a vibrator or sex where there is friction and it moves back and forth.

If it was so rough that the membranes got damaged, it would definitely be too rough. But I think it would be the sphincter that would get most affected if the sex were too rough.

Would you then get blood poisoning?

But when you take a dump then sometimes it also bleed a bit and nothing bad happens.

Yes, blood poisoning is very unlikely.