Introduction

The origin of viruses is not known. It has been hypothesized viruses may have evolved from DNA or RNA nucleic acid components of host cells that became able to replicate autonomously and independently, resembling genes that have acquired the capacity to exist on their own (Brooks et al. 2010). There is no evidence that viruses evolved from bacteria, though other obligately intracellular organisms, e.g., rickettsiae and chlamydiae, presumably did so (Brooks et al. 2010).