The primary distinction between a virus and bacteria lies in their structure and reproductive methods. Viruses are acellular entities composed of genetic material encased in a protein coat; they require a host cell to replicate and cannot carry out metabolic processes independently. Bacteria, conversely, are single-celled organisms with a complex cellular structure, including a membrane and cell wall, and can reproduce independently through binary fission. While bacteria can be beneficial, serving essential roles in digestion and ecosystem balance, viruses can cause diseases in various organisms, leading to significant health impacts.