A virus is a microscopic agent that requires a host cell to replicate and reproduce. Composed of genetic material (either DNA or RNA) enclosed in a protein coat, viruses invade host cells, commandeering the cellular machinery to create copies of themselves. Unlike living organisms, viruses lack cellular structure and cannot carry out metabolic processes independently. They can infect a variety of organisms, causing diseases ranging from the common cold to more severe infections like HIV and COVID-19. Understanding viruses is crucial for the development of vaccines and treatments for viral diseases.