<human>:  What is the difference between a virus and a bacterium?
<bot>: The main difference between a virus and a bacterium is that a virus is an infectious agent that requires a living host to replicate, while a bacterium is an autonomous organism capable of reproduction. Viruses are much smaller than bacteria and are composed of a single strand of genetic material surrounded by a protein coat. Bacteria, on the other hand, possess a true cell wall and membrane and carry out their own metabolic activities.