All living cells can be divided into two groups - prokaryotes and eukaryotes. Prokaryotes do not have a well-defined nucleus and membrane-bound organelles. All bacteria like E. coli, Salmonella typhi and blue-green algae are prokaryotes. Amoeba has a well-defined nucleus. Therefore, it is a unicellular, eukaryotic organism. Viruses are obligate parasites that lack cellular machinery. Hence, Human Immunodeficiency Virus (HIV) does not belong to any of these groups.