Cell
The only kinds of prokaryotic organisms are bacteria and archaea. Prokaryotic organisms evolved before eukaryotic organisms, so at one point the world consisted of nothing but prokaryotic organisms. There are also viruses, which are biological organisms. They always reproduce inside the cells of other organisms. They cause some important diseases. Viruses are made of RNA, or DNA, and protein, and they reproduce themselves inside the cells of bacteria or eukaryotes.