Bacteria are prokaryotes, thus they are simple, single celled organisms. They are found pretty much everywhere and replicate rapidly via binary fission (click for more information). The majority of bacteria in the world actually play a very important role in the ecosystem.
Irritation can be caused by the toxins the produce, interfering with cellular processes. Bacteria may also release enzymes which break down our cells and tissue.
Viroids are similar to viruses however much simpler and smaller ( 1/10 the size of the smallest virus) in design. Unlike a virus they don’t have a protein coat, or any other enzymes. They are simply a single stranded circular piece of RNA.