Living creatures are divided into two key groups: the prokaryotes, where the DNA in their cells is not enclosed in a membrane-bound nucleus and the eukaryotes where the cells do have their DNA enclosed in a nucleus and often contain other membrane-bound organelles as well. The eukaryotes include fungi, protozoa, algae, plants and all multicellular animals. Prokaryotes include bacteria and the mysterious archaebacteria which are prokaryotic in general structure but also share some characteristics with eukaryotes. Viruses are akaryotic because they are non-cellular. Prions are not micro-organisms but they are studied by microbiologists.