I think if you stick to vaginal secretions you should be fine. But faeces? I'm not sure. Look it up on google, there should be something there for you.
You ask any dentist and he will tell you that there is more bacteria in your mouth then in your anus. The mouth an nose in particular are designed to deal with bacteria. Our ancestors pulled food right out of the ground and ate it. They did not was their hands when they took a dump. You can probably stick a **** in you mouth and not get sick. The stomach is highly acidic and kills a lot of bacteria. There is a reason why you have an urge to pee after sex... the uric acid kills bacteria. If you ever cut yourself out in the woods an need to clean the wound, you can use your own pee. The problem with bacteria and viruses is when they gets directly into your bloodstream. Still the body can deal with most of them. Panty sniffing infection? No way. That is a far cry from the real **** plumbers and cowmen deal with every day. The body can handle it. It has been doing so for thousands of years before advanced plumbing systems.
I think you are risking bacterial infection or something of that sort. But also that could depend on the source. You say you prefer soiled underwear, so I assume it would have feces. Depending on how heavily soiled would probably relate to how big of a risk. To be safe, could you stick with panties that do not have feces, only secretions? Would that work the same for you?
That is really something I do not know the answer to. Bacterias effect each person in a different way. I really can not see it being a big issue, but like everything there is most likely a slight chance.
i know for a fact there wont be a problem with stds but with the bacteria is it way to harmfull? or can my imune system handle it
I am sure there is a chance of some bacteria issues but I would doubt there would be a problem with stds.