My science background is rusty, but wikipedia confirmed what I thought:
Bacteria - single-celled organism, your body hosts billions of them, most benign and necessary, but some certainly may affect fertility. Some are known to cause miscarriage (eg listeria). Some will just outright kill you, and thus affect your fertility.
Virus - not even unicellular - they are submicroscopic and unable to reproduce without a host cell. Certainly viruses can affect fertility, for instance HPV can lead to cervical cancer which has an impact on fertility, and, like bacteria, can just outright kill you. Ebola, for example, has a pretty huge impact on fertility.
Parasite -
multicellular organism, usually animal. Parasites are typically worms or mites that live in the host's body and use the host's body for sustenance. They are not usually directly responsible for infertility, but may contribute by robbing the host's body of nutrients required for fertility. Or they can just kill you, too.
Yeast - unicellular but can form interrelated colonies or strings. To the extent that yeast colonizes and irritates the vaginal tract, it probably does have an impact on fertility. Yeasts are in the fungus family, and there are possibly other unicellular fungi that can affect humans. Multicellular fungi are frequently tasty, and served fried with butter.
The other kind of organism? cell? thingy? that can infect humans is a prion, a malformed protein which can theoretically induce other proteins to kink up weirdly, causing neurodegeneration. This is the theoretical cause of Creutzfeld-Jacob disease (mad cow) but hasn't been conclusively proven, and there is also the possibility that there is an as-yet unidentified causative agent for the prions. I dunno about the fertility impacts of prions.
Does that help? Are you concerned about beasties interfering with your fertility?
