Bacteria belong to the monera kingdom, alongside plants, animals, fungi, and protists, and are therefore considered living creatures.

Viruses, on the other hand, are closer to collections of molecules that operate cooperatively to replicate, requiring that they co-opt a host’s cellular molecular machinery for their own purpose.

A virus may be adapted to live in a range of biologically similar organisms, in some cases being so specific as to be able to infect only one type of animal host or almost all animals in a given class in other cases. Viruses may also utilize living organisms outside of the animal kingdom as hosts, including plants, fungi, and bacteria, the latter of which is known as a bacteriophage.

The bacterial genome varies in size from several hundred thousand to over ten million base pairs, while the viral genome generally bears less than one million, with the smallest known RNA virus genome being only 1,700 base pairs. All bacteria utilize DNA that is stored in the cytoplasm in either a linear or circular form, and they engage in transcription to produce messenger RNA that is then translated into functional proteins within the cell.

Viruses also stow their genome in circular or linear forms, though it may be composed of single or double-stranded DNA or RNA, and they do not carry their translation machinery and must commandeer that of the host to generate viral proteins.