Resemble small bacteria, but like viruses they can only multiply by invading the cells of another life form.

Rickettsia

Explanation:

Rickettsia are usually considered as a separate group of bacteria. These microorganisms which multiply by binary fission inside their host cells are a group of obligately intracellular gram-negative mircoorganisms, having cell walls that look exactly as that of a typical bacteria cell wall. They possess no flagella. They are carried as parasites such as lice and tcks, and also cause diseases such as typhus.